XVID: XviD

Quick Solution:
New: Windows Media Player 12 in Windows 7 can play Xvid contents without any additional software.
Windows Users:
32-bit: http://www.koepi.info (Koepi’s binaries)
64-bit: http://celticdruid.no-ip.com/xvid/ (Celtic Druid’s binaries)
Mac Users:
http://n.ethz.ch/~naegelic/download/ (get the binary)

More Info:
XviD is an open-source MPEG-4 alternative to DivX.
At one point Project Mayo split two ways. One way went commercial (DivX 4, 5, 6, 7) and the other remained open source (XviD).

XviD can decode DivX 4 and up contents. It can also decode videos with fourcc mp4v.

Xvid normally uses the fourcc XVID, but the encoder gives the option to label the output video with fourcc DIVX or DX50 (this is not a recommended practice, but can be useful if the ouput needs to be read in a DivX-certified player that doesn’t recognize XVID).

If you go to http://www.xvid.org you will find the source code, which is useless to anyone who’s not a programmer. What end-users need are binaries. Many different people distribute xvid binaries and there is no single authority. We included links to some of the more popular builders' sites. If for some reason those builds don’t suit your needs (perhaps because you’re on a non-PC platform, or seek special builds optimized for specific processors), feel free to find others through a search engine query.

Notes:
- DivX 5, 6 and 7; and 3ivx can decode XviD contents.

Keywords: Xvid, XVID, xvid

Some links to other xvid binaries builds.
If you know of more, or if a link dies, feel free to post a reply to this thread.
http://www.koepi.info (Koepi’s builds)
http://nic.dnsalias.com/xvid.html (Nic’s builds)
http://celticdruid.no-ip.com/xvid (Celtic Druid’s builds)
http://jawormat.republika.pl (Jawor’s builds)
http://n.ethz.ch/~naegelic/download/ (build for the Mac)
http://www.insaneness.com/xvid.html (Bourke’s builds - for the Mac) (dead)
http://umaniac.leffe.dnsalias.com (uManiac’s builds - somewhat out of date)