VLC (VideoLan) is a cross-platform universial media player. It supports a variety of different inputs, including DVD, VCD, MPEG, AVI, WMV, MP4, and MOV. It has full subtitle support, as well as built-in video filters. There are skins for VLC available at the developer website.
One of the most useful features of VLC is playing incomplete video downloads before they finish. I can start watching a movie as the rest of the data trikles in. VLC plugins are available for Firefox and Mozilla.
You can install VLC through the command-line with the following command: