Storm Raven said:
Disarm: opposed d20 roll.
Trip: touch attack followed by an opposed d20 roll.
Grapple: touch attack, followed by an opposed d20 roll.
Overrun: move, followed by an opposed d20 roll.
Aid Another: a d20 roll against DC 10.
Bull Rush: move, followed by an opposed d20 roll.
Feint: an opposed d20 roll.
Sunder: an opposed d20 roll.
Charge: a modifer to your attack roll and AC.
Two-Weapon Fighting: an additional attack plus a modifier to your attack roll.
Throw Splash Weapon: a touch attack.
Most of these have rules remarkably similar to one another. Most (other than charge and two weapon fighting which are basically just modifiers) involve either a d20 roll against a static DC, or an opposed d20 roll.
Disarm:
melee weapon
1) attack of opportunity
2) opposed d20 roll (attack roll, +4 large weapon, +4 larger creature, -4 light weapon)
3) if fail, opposed d20 roll
OR not melee weapon
1) attack of opportunity
2) opposed d20 roll (attack roll, +4 large weapon, +4 larger creature, -4 light weapon, -4 not a melee weapon)
Trip:
not using melee weapon
1) touch attack
2) attack of opportunity
3) opposed d20 roll (+Str attacker, +Str or Dex defender, +4 per larger creature category, -4 per smaller creature category, +4 more than two legs defender)
4) if fail, opposed d20 roll
OR using melee weapon
1) opposed d20 roll (+Str attacker, +Str or Dex defender, +4 per larger creature category, -4 per smaller creature category, +4 more than two legs defender)
2) if fail, opposed d20 roll or drop weapon
Grapple:
1) attack of opportunity
2) touch attack
3) opposed d20 roll (BAB + Str + size)
4) damage
5) move
Bull Rush:
1) move
2) attack of opportunity (special exception, getting damaged does not stop the bull rush)
3) opposed d20 roll (+Str attacker, +Str defender, +4 per larger creature category, -4 per smaller creature category, +4 more than two legs defender, +2 charging)
4) move more if able and desired
Note: The order of Grapple for AoO and Touch Attack is opposite of Trip.
Note: The opposed d20 rolls use different mechanics (BAB vs. attack rolls vs. opposed ability rolls).
Note: Trip uses different mechanics if you are using a melee weapon or not, and Disarm uses different mechanics if you are attempting to disarm a melee weapon or not.
Note: Bull rush has two special exceptions to the rules. You do not get stopped from a damaging AoO and the opponent does not get to use his Dex to oppose you. Nor can he avoid you like he can with a Overrun. There is no chance of just letting him rush past you, he either bounces off, or you bounce back. NO REQUIRED TOUCH ATTACK. He automatically "touches you".
Note: Charge is limited to a straight line, but Bull Rush and Overrun are not.
Note: This does not even go into the page of rules on what happens if you are successfully grappling.
Just look at Disarm versus Sunder where they should be virtually identical rules except for the damage. Sunder uses different rules for non-weapons/non-shields than Disarm. Why? Nobody knows.
I could go on. The only similar thing at all for most of the special attacks is that you roll a D20.