Hey, I just found something cool that I've never seen mentioned before...the Dung Snake!
This snake from Serpent Kingdoms 83 is mentioned as a creature that a spellcaster can take as a normal (not improved) familiar, granting +2 to saves against disease.
While it's obviously kinda gross, the cool thing about it is that the Dung Snake is a 3HD medium-sized animal, which I don't believe any other normal familiar is. It also has a speed of 20 climb/swim/land each, 19HP and a natural Fast Healing (1) and Improved Grab+Constrict+Multigrab, and scent, along with very high ranks in the scouting skills - so it's a decent tank, scout, and crowd control option! And since it's found all over Faerun, especially in Undermountain, it's not particularly rare either!
The reed snake and trewarm are also listed as possible familiars, though both of them are a bit weak.
Edit - also, since I can't resist a good poop joke: Holy crap! The Dung Snake is totally the Shit!