VLC is an excellent media player that plays pretty much everything. And I mean everything. You can play the new file types (OGM, Matroska) and older ones (AVI etc). You dont even need those extra bulky codec packs because they already come embedded in the player! :D :D
I use this myself, and I find it is lighter than Windows Media player or anything else I have tried. It can play DVDs and VCDs as well. I also watch alot of anime and this media player takes the cake :D
Go to the official site for more details: videolan.org