This one works really well.
http://www.mbpninjaaffiliate.com/. It's a paid script, but worth it imho. Can easily change out codes. Hides the aff too so your link looks like
http://www.domain.com/sponsor/ vs.
http://www.sponsor.com/gallery/user=affcode?312. Looks a lot cleaner. Also gives you click stats. Search google for coupon codes.