For more comprehensive codec packs, try out:
http://nimo.titanesk.com/modules/news/
http://www.axisnova.com/articles/k_lite_codec_pack.shtml
http://www.doom9.org/index.html?/software2.htm
http://home.hccnet.nl/h.edskes/finalbuilds.htm

The last link also has a copy of 'Real Alternative', which lets you play RealMedia w/o installing the attrocious Real Players.

For WMP alternatives:
SASAMI2k: http://sasami2k.new21.net/
Media Player Classic: http://sourceforge.net/projects/guliverkli

I still use the former most of the time despite the lack of updates. MPC gets better and better though.

BTW, DivX *is* MPEG-4, or at least an MPEG-4 implementation (it started out as a hack of Microsoft's MPEG-4 v3 codec for an AVI container, but has since been re-written from scratch).

BTW, in a recent doom9 shootout, XviD came out on top: http://www.doom9.org/index.html?/codecs-203-1.htm

Apple's MPEG-4 wasn't tested (it's not that good IMO) although Sorenson's SV3 is definitely competitive w/ everything out there. Quality-wise though, the WM8+ codecs are probably the best out there right now.

MPEG-4 rocks hardcore. I use it to compress all my video for the web now.

Thanks for the details Leonard, I forgot about Xvid. And hello Mr. Kahn, didn't know you checked out this little blog!