I like the DivX 'Playa'. I've found it works really well for most media formats wmp supports, and isn't so bulky. One thing I noticed is that sometimes it'll be quirky about long or non-standard filenames (ex: thisvideo-[DivX-Rip]-lol.s403.avi). If you just rename the files, most of the time you can get stuff to play that was erring out before.Here's the link:
http://www.divx-digest.com/software/playa.html