Making your own
http://www.sherlocksoftware.org/
for n00bs i reccomend:
PetzA (basically one big cheat code for petz 5. It also makes babyz work on vista and xp!)
Pets Workshop (MAKE COPYS and edit them. DONT ask me for help. Ill ignore you.)
for more experienced pet owners:
Tinker (no visual, mostly code)