Paragonistic Initiate (CC) add quite a few stuff while Flanking. Notably, the Tactical Combat (Spell Disrupter) ability, that is essentially Mage Slayer, except only while Flanking, and without -4 CL. Interestingly enough, you can take this PrC as your 4th class level, well before any other PrC, which means it's cheap to get.
I gotta say that playing with a mid-level Factotum/Chameleon/Warblade along with my Marshal/Paladin/Paragonistic Initiate/Swiftblade/Sorcerer Gish was fun. He had +11 INT, while I had +13 CHA. End Result = +24 Damage to his every attack, for free. Got scary quick.