Improved Sunder( Player's Handbook v.3.5, p. 96)
[Fighter Bonus Feat, General]
You are skilled at attacking your opponents' weapons and shields, as well as other objects.
Power Attack (PH) , STR 13,
When you strike at an object held or carried by an opponent (such as a weapon or shield), you do not provoke an attack of opportunity (see Sunder, page 158). You also gain a +4 bonus on any attack roll made to attack an object held or carried by another character.
A fighter may select Improved Sunder as one of his fighter bonus feats (see page 38).
Without this feat, you provoke an attack of opportunity when you strike at an object held or carried by another character.