Pebble Underfoot( Dragon Compendium)
[Fighter Bonus Feat, General]
You have learned a special combat technique useful for bringing down creatures much larger than you.
Improved Trip (PH) ,
When attempting to trip a creature at least two size categories larger than you, you gain a +4 bonus on opposed trip checks. If you fail to trip the creature, it cannot immediately attempt to trip you.
A fighter may select Pebble Underfoot as one of his fighter bonus feats.