FFDShow is a very good, fast and comprehensive video decoder for decompressing DivX, XviD, WMV, MPEG-1 and MPEG-2 files. Using libav codec from ffmpeg project, it post processes code from mplayer to improve visual quality of low bit-rate movies.
The program is strongly recommended especially with “stammering” movies (when using a weak machine) or when having problems with playback of the movies saved in DivX 3.x and 4.x format. Thanks to precise algorithms for enhancing decoded video, FFDShow creates a clear, nearly useless file.
When it comes to its drawbacks: the application has not been updated for a while, so it may seem prehistoric to some of the users. There is also the ubiquitous install-me-and-I-will-freeze-your-browser toolbar, but it's quite easy to tick it out before installing the application. Some users are also cantankerous about the confusing and complicated configuration options, but it is next to impossible to find a machine that would need fixing before playing.
As a freeware program, FFDShow is definitely the best codec pack around.