Dungeons and Dragons
   Both the second and third edition systems use hit points to represent character health. The total hit points a character has depends on their level and the hit die granted by their class. In second edition, the hit die are as follows: d10 for warrior classes, d8 for priest classes, d6 for rogue classes, and d4 for mage classes. For the first several levels, a character's hit points are determined by rolling the appropriate hit die, adding in a Constitution modifier, and adding the result to the current hit point total. At higher levels, only 1 to 3 hit points are added depending on the base class.

Third edition doesn't divide classes into four base classes like second edition does; each class has its own hit die, though the choices for each class are similar to the second edition system. The system for determining hit points is similar, though; the difference is that hit dice are rolled for every level, not just the lower levels, and the first character level automatically grants the maximum possible hit point total for the hit die and Constitution modifier.

Damage is naturally represented by a loss of HP, with healing represented by the restoration of HP. In third edition, characters are disabled at 0 HP, unconscious and dying at -1 to -9 HP, and dead at -10 HP or lower. In second edition, many dungeon masters used the same system, except that 0 HP was considered unconscious and dying as well. Damage is usually applied by damage rolls using one or more of the polyhedral dice with a Strength bonus adding it. Many spells offer a saving throw which halves the damage when successful.

Advantages: The primary advantage of this system is simplicity. It's easy to keep track of HP and doesn't provide any penalties to keep track of during combat, making the overall flow simpler.

Disadvantages: The HP progression is extremely pronounced; HP can potentially double between first and second level. Thus, low-level characters can often be killed in a small number of hits where high-level characters can often withstand a great many, sometimes making battles last considerably longer (and intolerably long at times). Additionally, there is only one real intermediate grade of damage; while this is listed as an advantage for the simplicity, it is a disadvantage at the same time. In addition, the disabled category only happens at 0 HP; this is a disadvantage since it isn't realistic for a character with low HP to fight as well as a healthy character. Also, this means that damage rolls are different than skill rolls, making them a special case.

GURPS
   GURPS is another system which uses HP to represent a character's injury status. (These remarks apply to the fourth edition system; I do not know how they apply to previous editions.) The character's ST (Strength) attribute is the primary means of determining total HP, as total HP usually equals ST. The character can use character points to improve or lessen HP further without affecting ST, though. The character also has a HT (Health) attribute which is important at negative HP levels.

Damage is applied by rolls based on weapon damage or ST. GURPS has more intermediate levels of damage, though. At one-third of total HP and lower, Dodge and move are halved as a result of injury. At zero HP or lower, the character stays conscious only through willpower; he must make successful HT rolls to stay conscious. When the character has as many negative HP as his HP total, the risk of death occurs. On a failed HT roll, the character dies. The character must make an additional HT roll to avoid death at each negative multiple of his total HP, and automatically dies when he has negative HP equal to or exceeding five times his HP total.

Advantages: This is a simple, easy-to-use system that has reasonable effects for intermediate damage levels. The one-third level threshold means that a "disabled" condition is more likely to happen in a realistic way. The HP progression is also more reasonable.

Disadvantages: HP totals will often be around 10 to 20, so even a one-point loss can be fairly significant. Also, the use of HP means that this system again has damage rolls based on a different mechanic than skill rolls, so they are again a special case.

Star Trek RPG (Last Unicorn edition)
   This system does not technically use HP, but it is essentially the same as a HP-based system as it relies on points of damage inflicted. These are assigned either by a fixed number, as with untrained, unarmed attacks causing damage equal to the character's Fitness attribute plus Strength edge, or by a die roll and bonus, as with most other attacks. The amount of points a character can withstand without feeling ill effects from damage, called Resistance, is equal to his Fitness attribute plus Vitality plus armor modifier. Each time the character's damage exceeds a specific multiple of his Resistance, he enters new wound category.

Characters start at Healthy. At above one times Resistance, they become stunned and suffer penalties of 1 point on die rolls. Above two times Resistance, they become injured, unable to act for a round, and still suffer the 1 point penalty. Above three times Resistance, they become wounded, unable to act for a round, and start suffering 2 point penalties. Above four times Resistance, they become incapacitated, rendering them unconscious for 2d6 minutes and unable to act until receiving medical attention. Above five times Resistance, they are near death, and above six times Resistance, they die.

Advantages: The system is pretty realistic with its damage categories, and the simple system of having wound categories based of damage exceeding multiples of resistance is easy to remember.

Disadvantages: Since Resistance is almost always a single-digit number, there's a very extreme difference between low Resistance and high Resistance. Characters with low and even average Fitness and Vitality might be easily killed by a single hit due to the high damage rolls. While even high-resistance characters would likely be killed by two or three hits, low-resistance characters might not even have a chance. This isn't realistic, as virtually any character should be able to survive a glancing blow or grazing hit.

Star Wars d6
   This system relies on its skill roll system for handling damage. There is no HP or point system of any kind; instead, injury categories are used. A character's health or fortitude is represent by their Strength attribute, which is a die code, often 2D, 2D+1, 2D+2, or 3D. Armor often adds a die or two to Strength for the purpose of resisting damage. (The system uses only six-sided dice, so the "6" in "d6" is dropped.) Damage potential is represented the same way, and for unarmed attacks, damage occurs on a Strength vs. Strength roll.

No damage occurs if the target's Strength roll beats the damage roll. If the damage roll equals or exceeds the target's strength roll, the following occurs based on the difference between them: target is stunned on a difference of 0 to 3, wounded on a difference of 4 to 8, incapacitated on a difference of 9 to 12, mortally wounded on a difference of 13 to 15, and killed on a difference of 16 or more. A character already having one of the injury conditions is usually upgraded to the next condition if the damage roll indicates a lesser or equal injury condition; i.e. a stunned character becomes wounded if stunned ahead. The exception is wounded, since there is a wounded twice injury condition. Lesser conditions impose skill penalties by taking dice away from skill rolls and usually involve a character falling prone and possibly unconscious for a time. Mortally wounded characters are naturally unconscious until healed.

Advantages: This is a pretty realistic system affording the possibility of killing on a single hit in even most situations, even though it's not always likely. Damage is also described in English terms, not numbers.

Disadvantages: There are enough injury categories that it can be a little difficult to remember all of them and their effects, and the incapacitated condition calls for a 10d6 roll to determine how many minutes a character remains unconscious, which is a pretty extreme roll. Additionally, light amounts of damage cannot be handled by this system; a tough character might take several hits to even be stunned due to the need for the damage roll to at least equal the target's Strength roll. High-enough Strength or armor is almost near immunity to low-damage weapons that should still logically cause damage, even if gradually. The other problem is that it is easy to be killed.

My own system
   The system I've developed for my tabletop RPG has similarities to both a HP-based system and a skill-roll-style system. The damage categories are based upon trauma and stun levels. Trauma levels of less than 60 are stable. Trauma levels between 60 and 99 still allow the character to act normally, but his condition is no longer stable and he is getting worse. At a trauma level of 100, the character's vital functions stop and they fall unconscious. At 150, the character brain dies, and at 200 or more, the character's body is essentially destroyed. Stun levels are similar, with characters suffering no ill effects at stun levels less than 60. At stun levels of 60 to 99, characters start suffering penalties to their actions (-1, -3, or -5 depending on the exact stun level). At 100 or more, the character is unconscious.

Unlike HP-based systems, however, the value rolled on a damage roll is not the amount of damage applied. Instead, a power test for the weapon or attack is rolled against the target's fortitude. A critical failure inflicts no damage. A failure inflicts only 1, 3, or 5 points of damage depending on how many points the roll failed by. A success inflicts 10-50 points of damage depending on how many points the roll succeeded by. A critical success inflicts 60 or more points of damage, and in extreme cases, can kill a healthy character in a single hit. Trauma and stun damage rolls are made separately.

Note: I am currently considering some changes to critical failures and critical successes that may involve minor changes to the damage system, but the basic idea won't change.

Advantages: Moderately realistic, as characters can die in a single hit, but it's unlikely as the damage roll has to be not just a critical success, but a very high critical success. At the same time, characters will tend to go down in a few hits unless their fortitude is extremely high. While results that cause no damage are possible, they are extremely unlikely, so even gradual damage can add up.

Disadvantages: It's complex as it incorporates some features of all the previously described damage systems. It's also probably more random than is realistic; virtually any hit can be either almost ineffectual or absolutely devastating, though results tend to be in the middle of the spectrum (with the exception of heavy weapons).