virtualdub probably THE best avi manipulator on the M$ Windows platform.
Donald Graft's site lots of virtualdub and avs filters, plus a VERY good links section
Koepi's Xvid binaries Get your xvid installer binaries here. Also links to nic and Umaniac builds.
divx digest a site that hosts a lot of divx related files.
AVISYNTH a must have for the advanced encoder. Script based video frame serving into virtually any avi capable application.
Doom9 forums where many virtualdub, xvid, gordian knot, and other developers of open source tools for video encoding spend their spare time.