The K-Lite Codec Pack (for Windows only) will give you the ability to play almost every multimedia file on the face of this earth. It has support for a lot of proprietary and open source codec, it has a media player, and it has some tools to help you do some encoding. It also includes a DVD decoder so you can play DVD with Windows Media Player after installing this Codec Pack.
VideoLAN Client is a multi platform application that can be used to play a lot of multimedia content (AVI, MPEG, VCD, DVD) and can also stream the content to your network or to the internet.
This is one of the first application that provides Linux with the ability to play almost any multimedia file. Mplayer can be used to play Video file (almost any file, if you installed the codec), Audio CD, Video CD, and even DVD. Windows version of Mplayer are also available.
Mplayer can use Windows DLL Codec on Linux, very interesting and useful (but some people object on its legality.
Another multimedia player for Linux. From the manual: Xine plays MPEG system (audio and video) streams, mpeg elementary streams (e.g. .mp3 or .mpv files), mpeg transport streams, ogg files, avi files, asf files, quicktime files, (S)VCDs and DVDs. Xine doesn't use Windows DLL.
VirtualDub can open Mpeg and AVI files and convert them to another file. While converting, Mplayer can apply filters to the video stream (for example to flip the display or to display a logo). VirtualDub only exists on Windows but is runnable on Linux using Wine.