K-Lite Codec Pack 2.35 Full
Installation instructions:

It is highly recommended to first uninstall all existing codec related packages before installing this package. This way your system stays clean and potential problems will be avoided. You can remove them in Start Menu > Settings > Control Panel > Add/Remove Programs (Software).

Features & Notes:

Media Player Classic [version 6.4.8.2]
This is a full-featured player. Has internal decoding support for DVD, MPEG-2, DTS, Matroska, OGM and some other formats. It plays QuickTime and RealMedia (if those codecs are installed).

DivX Pro [version 5.2.1] [Decoding]
This is required for playing DivX movies. DivX is the second most popular format.
DivX Pro [version 5.1.1] [Encoding]
This is required for encoding movies in the DivX format.
XviD [version 1.0.3 build 2004-12-20] [Decoding]
This is required for playing XviD movies. XviD is currently the most popular format.
XviD [version 1.0.3 build 2004-12-20] [Encoding]
This is required for encoding movies in the XviD format.
3ivX Pro [version D4 4.5.1] [Decoding]
This is required for playing 3ivX movies. Also supports various other MPEG-4 variants.
Required for opening audio and video streams in a MP4 container (.mp4 files).
3ivX Pro [version D4 4.5.1] [Encoding]
This is required for encoding movies in the 3ivX format.
Windows Media 9 VCM [version 9.0.1.369]
This is required for playing movies and audio that are in the Windows Media 9 format.
Windows Media [version 10.0.0.3646 / 8.0.0.4487 / 8.0.0.4000]
This is required for playing movies and audio that are in the Windows Media format.
On2 VP3 [version 3.2.5.0]
This is required for playing VP3 movies. These are NOT very common.
On2 VP6 [version 6.2.0.10] [Decoding]
This is required for playing VP6 movies. These are NOT very common.
On2 VP6 [version 6.2.0.10] [Encoding]
This is required for encoding movies in the VP6 format.
Ligos Indeo XP [version 5.2820.15.58]
Required for playing movies that have the Indeo 5 format.
Intel Indeo [version 4.51.16.2]
Required for playing movies that have the Indeo 4 format.
Intel Indeo [version 3.24.15.03]
Required for playing movies that have the Indeo 3 format.
Intel I.263 [version 2.55.1.16]
Required for playing movies that have the I.263 format.
huffyuv [version 2.2.0]
Required for playing movies that have the huffyuv format.
Required for encoding movies in the huffyuv format.
Huffyuv is a lossless (no quality loss) video compression format.
MS MPEG-4 [version 4.1.0.3927]
Required for playing (and encoding) movies that have the MS MPEG-4 format.
DivX ;) MPEG-4 Low and Fast motion [version 4.1.0.3927]
This is an old version (v3.11) of DivX. Only required for those who want to use it for encoding something. 
Cyberlink DVD decoder [version 6.0.0.1006]
This can be used to play DVDs with Windows Media Player, but only in combination with Windows XP.
Not required if you use Media Player Classic, because it has an internal DVD decoder.
Elecard MPEG-2 demuxer [version 2.0.84.30429]
MainConcept MPEG-2 demuxer [version 1.0.0.54]
Windows already has a MPEG-2 demuxer by default so normally you won't really need these.
Ligos MPEG-2 decoder [version 4.0.0.77]
Cyberlink MPEG-2 decoder [version 6.0.0.1006]
MainConcept MPEG-2 decoder [version 1.0.0.56]
Required for playing MPEG-2 (and SVCD) movies. Ligos and MainConcept will also decode MPEG-2 audio streams.
Not required if you use Media Player Classic, because it has an internal MPEG-2 decoder.

Fraunhofer MP3 DirectShow decoder [version 1.9.0.311]
Required for playing movies that have a MP3 audio stream.
WMA DirectShow decoder [version 8.0.0.4487]
Required for playing movies that have a WMA audio stream.
AC3 DirectShow decoder [version 0.70b mod]
Required for playing movies that have an AC3 audio stream.
AC3 ACM decoder [version 0.7]
Required for playing movies that have an AC3 audio stream.
Vorbis DirectShow decoder (CoreVorbis) [version 1.0]
Required for playing movies that have a Vorbis audio stream.
AAC DirectShow decoder (CoreAAC) [version 1.0b9]
Required for playing movies that have an AAC audio stream.
Not required if you install the 3ivx AAC decoder.
3ivX Pro [version D4 4.5.1] [Decoding]
Required for playing movies that have an AAC audio stream.
Not required if you install CoreAAC.
3ivX Pro [version D4 4.5.1] [Encoding]
DirectShow encoder for AAC audio.
Nero Digital MPEG-1/2 decoder [version 1.0.4.6]
Required for playing MPEG-2 audio streams.
MusePack DirectShow decoder [version 1.0.0.3]
Required for playing MusePack audio files.
Voxware DirectShow decoder [version 1.0.0.12]
Required for playing movies that have a Voxware audio stream.
Monkey's Audio DirectShow decoder [version 1.00]
Required for playing Monkey's audio files (.ape).
Monkey's Audio is a lossless (no quality loss) audio compresion format.
DivX ;) Audio [version 4.2.0.0]
Some DivX 3.11 videos use this.
LAME MP3 Encoder [version 3.96.1]
This can be used to encode MP3 streams/files.

Ogg DirectShow splitter [version 0.9.9.5] and Vorbis DLLs [version 1.1.0]
Required for opening audio and video streams in an Ogg container (.ogm and .ogg files).
Not required if you use Media Player Classic to play these types of files.
Matroska DirectShow splitter [version 1.0.2.4]
Required for opening audio and video streams in a Matroska container (.mkv and .mka files).
Not required if you use Media Player Classic to play these types of files.
Matroska DirectShow muxer [version 1.0.0.9]
Can be used to mux audio and video streams into a Matroska container.
DirectVobSub (vsfilter) [version 2.33]
DirectVobSub is a tool for displaying subtitles. Your subtitle file should have the same filename as your video file (for example Movie.avi and Movie.sub) and should be in the same folder as your video file. Then the subtitles will automatically be displayed in most players.
Matrix Mixer [version 0.30b]
Matrix Mixer can be used to amplify the sound of a video file. It also can mix stereo sound to multiple channels (useful for 4.1 and higher speaker sets).
SHOUTcast Source [version 1.0.0.1]
This can be used to play SHOUTcast streams in any DirectShow enabled player.
Morgan Multimedia Stream Switcher [version 0.9.9]
Morgan Multimedia Stream Switcher can be used to switch between two audio streams in a video file that has two audio streams. These types of video files are not very common. BSplayer also has this functionality built-in. So if you use BSplayer, you don't need to install this.
DivX Anti-Freeze [version 0.4]    (CAUTION - not recommended!)
This prevents freezing of some players when playing a damaged DivX movie. This filter is not needed with the current version of DivX, only with old versions!
Warning! This filter causes problems with some programs like MovieMaker. It is not recommended to install this.

GSpot Codec Information Appliance [version 2.52 beta 1 build 2004-09-15]
GSpot is a tool that can tell you exactly which codecs are needed to play a certain video file. This is very helpful when you have troubles playing a video file.
FourCC Changer
The FourCC (Four Character Code) Changer is a program for changing the FourCC of a video file, which is used to identify which codec should be used to decode the file. This tool is for expert users only.
Bitrate Calculator
This can be used to calculate the bitrate to use when encoding a movie.