Although martial arts can already be protrayed with the use of skills and feats (Tumble, Power Attack, Improved Trip, etc.) the following rules give unarmed characters more combat options.
Striking with different parts of the body
Just as different weapons have different effects, so do unarmed strikes from different body parts. Any creature with the appropriate body part can perform these maneuvers. When using the full attack action you must use all of your attacks with the same body part. However, you can fight with two body parts as if fighting with two weapons. Additionally, monks can freely alternate the body part they are attacking with as they see fit. Keep in mind that without the Improved Unarmed Strike feat you provoke attacks of opportunity from armed opponents, and suffer a -4 penalty to attack rolls will attempting to inflict lethal damage.
Technically a punch is an attack made with a closed fist, but for the purpose of this rule any attack made with the hands or arms is considered to be a punch. A punch is the standard unarmed strike and is not modified in any way.
For the purpose of this rule a kick is any attack made with the feet or legs. A kick attack gains double your Strength bonus to damage. Kicks used as an off-hand attack gain your normal Strength bonus to damage instead of 1/2 your Strength bonus. If you do not have a Strength bonus or have a Strength penalty then the kick only inflicts 1 additional point of damage. However, kicks are unbalancing and harder to land, thus kicks suffer a -4 penalty to the attack roll.
A head butt is an attack made with the head, usually the fore head. A head butt has a chance of staggering an opponent. The target must make a Fortitude save (DC equal to the damage dealt) or be staggered for 1 round. For every size category the target is larger than you, it gets a +4 bonus to its saving throw. However, you have a chance of stunning yourself. You must make a Fortitude save (DC equal to the damage dealt) or be stunned for 1 round.
Any creature can attempt to perform these special moves with unarmed strikes. Monks can use their special monk weapons when performing these moves if they wish. Special moves tend to be sub-optimal choices in combat. You normally would not use the same move over and over again, only when the situation called for it. Keep in mind that without the Improved Unarmed Strike feat you provoke attacks of opportunity from armed opponents, and suffer a -4 penalty to attack rolls will attempting to inflict lethal damage.
When you are flanked by two opponents, you can split an attack to strike both opponents as an attack action. Creatures must strike with two punches, or two kicks (which ever unarmed strike they are currently using). A monk can use any unarmed strike or monk weapon as he sees fit. You must make a separate attack roll against each opponent at your current attack bonus, along with a -4 penalty to both attacks. This special move can only be performed once per round, however a monk can perform this special move with all of his attacks during a round.
As an attack action, you can spin around during an attack to gain momentum. You suffer a -10 penalty to the attack roll, but you inflict double damage on a successful hit. Performing this special move provokes an attack of opportunity. This special move can only be performed once per round, however a monk can perform this special move with all of his attacks during a round.
As a move action, you can jump before attacking your opponent. For every foot of clearance you gain a +1 bonus to your attack roll. Performing this special move provokes an attack of opportunity.
When using the full-attack action, you can move 5 feet between each attack. This movement provokes attacks of opportunity. Additionally, every 5 feet of movement is treated as a separate opportunity for opponents to make an attack of opportunity. You must use an attack action between every 5 feet of movement, but you need not move between each attack. You cannot move a total distance greater than your speed. You cannot take a 5-foot step during the same round that you perform this special move. You suffer a -2 penalty to all attacks when performing this special move.
When using an attack action or full-attack action, you can forgo your highest attack and gain a +2 shield bonus to Armor Class. If you also forgo your second highest attack then you gain an additional +2 shield bonus to Armor Class, and so forth. You must designate one opponent against which this shield bonus applies, or you can distribute this bonus among several opponents. For example, a 20th level monk could forgo his five flurry of blows attacks and instead gain a +10 shield bonus to Armor Class. He could then apply the entire +10 shield bonus against one opponent, or gain a +5 shield bonus against two opponents, or gain a +2 shield bonus against 5 opponents, or any combination that does not exceed a total of +10.
Combining Special Moves
You can combine special moves together to form more complex moves. When doing so, you gain all of the benefits as well as all of the drawbacks for each special move. For example, you could perform a jumping, spinning, split kick.
Martial Arts Feats
These feats are for use with the optional martial arts rules described above. A monk can select these feats as a bonus feat at levels 1, 2, and 6 instead of the normal bonus feats available. Fighters can select these feats with their bonus feats.
You are less reckless when attempting a particular special move.
Benefit: Choose one special move (such as jump attack). You no longer provoke an attack of opportunity when you perform this special move.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new special move.
You are an expert at blocking attacks.
Benefit: The shield bonus you gain for each attack you forgo is increased by 1.
Special: You can gain this feat multiple times, its effects stack.
You are better able to perform difficult moves.
Benefit: Choose one body part or special move that applies a penalty to the attack roll (such as kick, or split attack). The penalty to your attack roll is reduced by 2.
Special: You gain this feat multiple times. Each time you take the feat, it applies to a new body part or special move, or it stacks with the same body part or special move.
You are an expert with a particular fighting style.
Benefit: Choose one body part or special move (such as kick, or spin attack). You gain a +1 bonus to attack rolls when attacking with the chosen body part or special move. This bonus stacks with itself and the Weapon Focus (unarmed strike) feat. For example, a monk with Style Focus (kick) and Style Focus (spin attack) would gain a +2 bonus to attack rolls when performing a spin attack using a kick.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new body part or special move.