Rex's AVI Codec Pack

Rex's AVI Codec Pack is a collection of all common audio and video codecs that you will ususally need in order to playback most media files these days.
The package includes all common codecs such as XviD, DivX, ac3, mp4, 3gp, wmv, mpeg4 and many more.
During the installation you get to choose what codecs to install, or you can simply just stay with the recommended option which will install the most common ones and enable most audio/video playback.