Creature Labs--For me, this is the one that started it all. I discovered Creatures from the Creature Labs web page, one day while I was searching Google for virtual pets. The rest, as they say, is history.
Gameware Development Forums--The heart of the CC, it needs little explanation. (Down, due to lack of moderation, since 19 July 2008).
Creatures Caves--A great website with downloads, forums, and more. Of particular note is the recent addition of AquaShee's agents and tutorials.
Creatures Heaven--Vampess' site that features many things, among them the revolutionary CFEs(Creatures Full of Edits). A definite must-see.
Creatures Wiki Homepage--The definitive guide to Creatures, and the Creatures Community. Not to be missed.