Elemental templates from 3.0 Book of Templates (the original, not the deluxe one).
Best elemental templates I've seen, no breath weapons, smites, or spell abilities, just appropriate elemental traits and powers.
For WotC sources alone, hmm, 3.5 lycanthrope. Fixed a lot of problems with the 3.0 version and made it much more straightforward to implement. D20 modern vampire is a nice improvement on the standard D&D vamp too. Oh and the shade small template from tome of magic is nifty and real easy to add on.