XviD is a codec just like DivX. Most new movies and videos are being encoded with XviD. Right now 2 new codec versions have been released:
Koepi's XviD Codec Build 24062003-1 uManiac's XviD Codec Build 26.06.2003.1100 (My personal favourite) Click on Read More... to see what's new and download links.

Koepi's XviD Codec:
Download Link Latest development (unstable) binary: XviD-24062003-1.exe (389kb) What's New:
Fixed P4 issues and the use of SimpleIDCT when unchecking all CPU optimizations. Remember to switch IDCT to XviD when decoding with ffdshow. HomePage uManiac's XviD codec:
Download Link Latest Unstable Build 26.6.2003 11:00 What's New Home Page