The videolan.org team that develops VLC media player has after two years launched a new version of thier media player. It plays a very large list of different media formats including all popluar formats like MPEG4, and avi files. I was personlly turned onto VLC media player when I was looking for a media player that supports subtitles. I have been really impressed in the past how robust and easy to use this application is. It is definetly an application that everyone should know and will find extreamly useful.
The latest version of the VLC media player is 0.9.2, and is code named Grishenko. It can be downloaded here. For a list of new features look here. In addition the team is looking for help from developers and if you are interested look here.