There are quite a few, actually.
Here's some useful ones:
Overhead Thrust[General]
(Dcn p106)
PrerequisitesBase Attack Bonus +6
Strength 13
Power Attack
Close-Quarters Fighting
BenefitYou can use a Slashing or Piercing weapon to make an Attack of Opportunity against a foe using an attack
designed to batter you from above (Overrun, Trample, Power Dive, Dragon Crush). You cannot use this
feat if you are flat-footed or already grappled.
You gain a special attack modifier based on your opponent’s size:
Medium or smaller (+0), Large (+4), Huge (+
, Gargantuan (+12), Colossal (+16)
If your attack succeeds, you do triple damage.
Giantbane[Tactical, Fighter]
(CWar p111)
PrerequisitesMedium-size or smaller
Base Attack Bonus +6
Tumble: 5 ranks
BenefitYou may use the following 3 tactical maneuvers:
Duck Underneath – If you take a Total Defense action against a foe who is at least two size categories larger
than you, you gain an additional +4 bonus on your AC. If your foe misses on his/her attack, you may
make a Tumble check vs. DC 15 to move to the opposite side of your foe.
Death from Below – If you successfully performed a Duck Underneath maneuver the round before, you can
make an immediate single attack on the foe you ducked under. Your foe is Flat-Footed & you gain a +4
bonus on your attack.
Climb Aboard – If you are adjacent to a foe at least two size categories larger than you, you may make a
Climb check vs. DC 10 to climb onto your foe. As long as you are on your foe, he/she has a –4 penalty on
attack rolls to hit you & you are carried with him/her. Your foe may get you off with a Grapple check
opposed by your Climb check.
Clever Wrestling[General] (CWar p97)
(Dcn p103)
(Storm p92)
PrerequisitesSmall or Medium size
Improved Unarmed Strike
BenefitWhen grappling with an opponent greater than Medium size, you gain a bonus when attempting to escape a
Grapple or Pin. The size of the bonus depends on the opponent’s size:
Large – +2 Huge – +4 Gargantuan – +6 Colossal – +8
Cunning Sidestep[General]
(Dcn p103)
PrerequisitesSmall or Medium size
Improved Unarmed
Strike
Clever Wrestling
BenefitWhen your opponent is larger than Medium size, you gain a bonus on any opposed check you make to avoid
being Bull Rushed, Tripped, Knocked Down, or Pushed. The bonus depends on the opponent’s size:
Large – +2 Huge – +4 Gargantuan – +6 Colossal – +8
This feat is effective against the Large and in Charge feat.
The bonus from this feat does not stack with Clever Wrestling.
And here's some shit for midgets:
Underfoot Combat[General]
(RotW p152)
PrerequisitesSmall size (or smaller)
Tumble: 10 ranks
BenefitYou can move into or through a square occupied by a creature at least two size categories larger than you.
You do not provoke Attacks of Opportunity for doing so.
When you are in a square occupied by a creature at least two size categories larger than you, you gain the
benefit of Soft Cover (+4 bonus to AC) again all attacks (including those of the creature whose space you
occupy).
Confound the Big Folk[Tactical]
(RotW p153)
PrerequisitesSmall size (or smaller)
Underfoot Combat
Tumble: 10 ranks
BenefitYou may use the following 3 tactical maneuvers:
Knee Striker – When you occupy a square with a creature at least two size categories larger than you, the
creature is considered Flat-Footed against you and you receive a +4 bonus on rolls to confirm critical hits.
Underfoot Defense – When you occupy a square with a creature at least two size categories larger than you,
and you Fight Defensively, use Total Defense, or use Combat Expertise, any melee or ranged attack on
you has a 50% chance of striking the creature who shares the square with you (that creature does not have
a 50% chance of striking itself).
Unsteady Footing – When you occupy a square with a creature at least two size categories larger than you,
you may initiate a Trip attack on the creature you share the square with and not provoke an Attack of
Opportunity. You can add your choice of Strength or Dexterity modifier to you check (your opponent gets
the better of its Strength or Dexterity as usual). Your opponent does not get to add his/her size bonus to its
roll. If the Trip attempt fails, your opponent does not get to try to trip you.
Halfling’s Cunning[General]
(DR323 p29)
PrerequisitesCharisma 13
Commander
Small size or smaller
BenefitAll allies within 30’ receive a +1 Morale bonus on attacks vs. creatures at lease one size category larger then
themselves.
And my absolute favourite:
Swarm Fighting[General]
(CWar p105)
PrerequisitesSmall size
Dexterity 13
Base Attack Bonus +1
BenefitYou may share a 5’ hex with your Small-sized allies that also have this feat. Against a creature of at least
Medium-size, each attacker with Swarm Fighting who also has this feat (after the first) grants all of his/her
allies a +1 Morale bonus on the attack roll, up to a maximum of your Dexterity modifier.
For example, if 4 Halflings swarm fight an Ogre, each receives a +3 Moral bonus to attack.
As far as favoured enemy bonuses, there are quite a few magical items in The Magic Item Compendium that deal with augmenting class features.