To repeat some earlier posters, myconids, warforged, flumphs, nimblewrights, kenku, brain-in-a-jars, and catfolk are all high on my wish list. Maybe dracolich and grell, also.
Although they're more obscure, I like a number of the Tome of Magic (Deadly Dancer, Roving Mauler, Ephemeral Hangman) and Tome of Battle (Reth Dekala, Valkyrie) monsters. Also, let me toss in Luna Moths from Heroes of Battle.