Swarmfighting( Complete Warrior, p. 105)
You and allies with this feat can coordinate melee attacks against a single target and are adept at fighting side by side in close quarters.
DEX 13, base attack bonus +1, Small size,
You can occupy the same 5-foot square in combat with any other allied Small creature that also possesses the Swarmfighting feat at no penalty. When you engage a Medium or larger creature in melee, and at least one other ally with the Swarmfighting feat threatens the target, you gain a +1 morale bonus on the attack roll. This bonus increases by +1 for each additional ally beyond the first with the Swarmfighting feat that threatens the same target. The total morale bonus imparted to your attack roll cannot exceed your Dexterity bonus.