I don't know of any websites that present new familiars, but here's a list of books that do:
- Tome & Blood (WotC)
- Villain Design Handbook (Kenzer)
- Forgotten Realms Campaign Setting (WotC)
- Spells and Spellcraft (Fantasy Flight)
And one magazine:
- Dragon #280 (the sorcery issue)
There are probably others, but that's all I can come up with at the moment.