I've put together some stuff already, though it's mostly just A&EG and MiC stuff, plus some feats and items. Dungeoncrasher Fighter needs a special mention though.
Bonuses to Bull Rush:
Any +Strength effect
Size increases
A&EG stuff:
Battleaxe of the Bull, (pg 100): 32,310g. +2 battleaxe grants the Improved Bull Rush feat and if used in 2 hands any successful attack pushes a Large or smaller creature back 5 feet. . Of note is it's priced equivalent to a +4 weapon, meaning the bull rush + pushback price could either be a +2 bonus or is 24,000g if swapping specific weapon enhancements is allowed.
MiC stuff:
Brutal Surge weapon (pg 30): +1 melee weapon enhancement that allows the user to bull rush an opponent on a successful hit with all the creatures regular bonuses. +2 on the BR check if the weapon is used in two hands. User doesn't move with the defender on a successful BR. Useable Con mod + 1 times per day.
Armbands of Might (pg 72): 4,100g. +2 on all strength-based checks, +2 on melee damage if using PA for -2 on attacks.
Belt of the Champion (pg 73): 4,500g. One of Kord's relics. Equivalent to a Belt of Giant Strength +2 for certain alignments, and if the character fulfills the relic requirements it's +5 competence bonus on all strength checks. And +4 on saves against fear.
Brute Gauntlets (pg 83): 500g. Spend charges to gain a morale bonus on strength checks, strength-based skill checks, and melee weapon damage for 1 round.
Brute Ring (pg 83): 2,300. Make a ranged bull rush at +5 or your own modifiers that can affect incorporeal creatures. Works twice per day.
Helm of Battle (pg 111): 2,000g. Swift action to gain +2 untyped bonus on next on the next attack roll or ability check you make to attempt a bull rush, disarm, overrun, sunder, or trip attempt during your turn. Functions three times a day.
Third Eye Surge (pg 143): 2,100g. Swift action to spend charges and gain an insight bonus on Strength checks, Dexterity checks, Strength- and Dexterity-based skill checks, and weapon damage rolls for 1 round.
Torc of the Titans (pg 143): 3,300g. Swift action to gain a +5 morale bonus on Strength checks, Strength-based skill checks, and melee weapon damage rolls for 1 round. Useable 3 times per day.
Grafts:
Goring Horn graft (Fiend Folio pg 212): 8,000g. Grants Improved Bull Rush and allows the character to make a gore attack with the horn.
Traits:
Musclebound: +1 on strength-based skill and ability checks, -2 on dex-based stuff. Make sure the DM interprets it to NOT include initiative, because then it's almost certainly not worth it.
Feats:
Improved Bull Rush (PHB pg 95 or
SRD): +4 on bull rush attempts, don't provoke AoO. Fighter bonus feat.
Shock Trooper (CW 112): Directed Bull Rush allows you to push the opponent left or right in addition to backwards. Domino Rush allows you to make a trip attempt against two opponents if you bull rush one into the other. Fighter bonus feat.
Knockback (RoS 142): Requires being Large+ or have Powerful Build and Medium. Allows you to make a bull rush attempt upon a successful Power Attack and use the Power Attack attack roll penalty to boost the bull rush check. Fighter bonus feat.
Combat Brute (CW 110): Advancing Blows gives you a +1 bonus on all attack and damage rolls for each square you moved the foe back. Fighter bonus feat.
Driving Attack (PHB2 78): As a full-round action, use a piercing weapon to make a bull rush that uses the character's total bonus on damage rolls in place of his strength modifier. If the check is high enough, the user can choose to make the opponent fall prone.
Classes:
Dungeoncrasher Fighter (DgnScp 10): Trade 2nd level feat for +2 AC against traps, +5 strength bonus on checks to break down doors, walls, etc, and if you bull rush an opponent into a solid obstacle you deal 4d6 + 2x strength bonus as bludgeoning damage. Trade 6th level feat to make it +4 AC versus traps, +10 to break stuff, and 8d6 + 3x strength on bull rushes.
Factotum (DgnScp 15): At 3rd level, gain Int to strength checks, which includes bull rush.
Overwhelming Attack Monk. Gain PA and Improved Bull Rush in two levels, and +2 on Intimidate. It's somewhat humorous that a monk gets PA at 1st level but can't actually use it, but it's mostly a means to an end.
Spells:
Telekinesis (PHB/SRD): Can be used for a bull rush, which led to some interesting chatter about ghosts taking Dungeoncrasher fighter. In place of strength the caster uses int, cha, or whatever their casting stat is.
Battering Ram (SpC 24): Sor/Wiz 2. Close range spell to make a bull rush against a target as a medium creature with 30 strength, for +10 on the mod. Can also be used on objects to try and move them.
Capricious Zephyr (SpC 43): Dru 3, Sor/Wiz 3. Spell moves around and if it goes into an occupied square it makes a bull rush with +6 mod. If it succeeds, the foe is moved in a random direction. A pushed creature must make a reflex save or fall prone.
Cyclonic Blast (SpC 57): Sor/Wiz 5. Make wind torrent that does up to 15d6 damage to anything in a 120 foot line and creatures that fail a reflex save are bull rushed for +12 and pushed back the maximum distance based on the check.
Forcewave (SpC 98): Sor/Wiz4. Bull rushes all targets within 10 feet at a +10 bonus.
Obedient Avalanche (SpC 148): Cold 9, Sor/Wiz 9. Creatures 20 to 40 feet away from spell's center and who fail reflex save are bull rushed by the bank of snow's +13 mod.
Rushing Waters (SpC 178): Dru 4. A burst of water with a 15 foot radius from a chosen point gets a +15 to bull rush anything in that area. If it succeeds, the creature must make a Ref save or fall prone.
Tidal Surge (SpC 220): Dru 6. 20 foot burst that does up to 15d6 damage and makes a +5 or +10 bull rush against applicable targets, but nothing else.
