Official DivX website

Official XviD website (no codec downloads here!)
Koepi's Media Development (XviD codec downloads)

Official 3ivx website

Doom9 (Lots of encoding tutorials here)
DVDR/SVCD/VCD Help (Encoding, converting and burning tutorials specifically for DVDr, SVCD and VCD)

GSpot Codec Information Appliance

AC3 Filter
Matrix Mixer

Project Guliverkli (Media Player Classic, vsfilter, Matroska splitter, RealMedia splitter, etc.)

VideoLAN (player) (Download)

BSplayer (player)

VirtualDub (encoding tool)

Monkey's Audio (lossless audio compression format)
Monkey's Audio DirectShow decoder

MSDN DirectShow

RareWares (Information about AAC, Ogg Vorbis and MPEG4)

Hydrogen Audio (Audio experts)

Exact Audio Copy (Best cd rip software)

Official Ogg Vorbis website

Matroska

LAME MP3 binairies

FourCC.org (All FourCC codes)