A huge compilation of Win32 codecs:

Video:
- Win32 VfW DLLs:
    Indeo Video 3.2, 4.1
    Microsoft MPEG-4 v1 & v2 beta
    Microsoft MPEG-4 v3 ( also known as DivX ;-) )
    Cinepak Video
    ATI VCR-2
    I263
    Quicktime 6 (includes 3ivX, ZyGo)
    Realplayer 9 (includes RV40)
    xanim (includes 3ivX, Indeo3/4/5)
- Win32 DirectShow filters, decompression-only support:
    Microsoft MPEG-4 v3 ( this decoder is slower than VfW one,
      but offers wider range of picture control features )
    Windows Media Video 7
    Indeo Video 5.0
    Motion JPEG ( using Morgan Multimedia shareware codec )
- Open-source plugins:
    Motion JPEG ( using libjpeg, very slow )

Audio:
- Win32 ACM DLLs, decompression-only support:
    Windows Media Audio ( also known as DivX ;-) Audio )
    MS ADPCM
    Intel Music Codec
- Open-source plugins, decompression-only support:
    PCM
    AC3
    IMA ADPCM
    MPEG Layer-1,2,3 ( compression into MP3 is also supported )
    MSN Audio
    GSM 6.1 Audio
- Win32 DirectShow filters, untested decompression-only support:
    Voxware Metasound
    ACELP.net

WWW: http://www.mplayerhq.hu/MPlayer/releases/codecs/