Impressive World, Dragons Den, Last Moon, Sanctum Of Eventide. c:
I play Last Moon, and it's definitely my favorite out of all the Impressive Title servers I've tried. There are also a lot of different playable species besides wolves and lions, like rabbits, deer, ferrets, bears, griffins, etc. I also hear that they're working on a horse model and a dragon model.
A word of warning about Last Moon, though...there's a teeny bit of swearing in that game and on the forum. I'd give it a PG-13 rating.