Can you make a trip attack (or other special attack) as
an attack of opportunity?
Yes, as long as you make the trip attack (or other special
attack) with a weapon with which you threaten the target. For
example, unless you wield a weapon that allows you to make a
trip attack, trip attacks are unarmed attacks; a character who
doesn’t threaten while unarmed couldn’t make an unarmed trip
attack as an attack of opportunity.
DND FAQ 3.5 08-06-30 P70