Someone who doesn't receive nearly enough credit in this regard is Gracyanne Barbosa. She is understandably much better known for her glutes and quads, but her ab definition is phenomenal. In fact, I think her abs are her most impressive feature, though I doubt if too many others would agree with that statement :)
Ana Cozar, Maggie Corso and Toni Wheeler are also worth noting (especially Ana). Among newcomers, there is Stephanie Rodriguez.
Probably the best set of abs I ever saw on a woman belonged to Donna Bramble, though she left the bodybuilding scene a long time ago.
Dana Linn Bailey? Tara Scotti? Someone else?
(Please don't include obvious steroid users in your responses -- that's cheating!)