Real Player is used to play back RM and RAM video/audio files, it can also play back a variety of other video/audio formats.
A freeware standalone media player that can play back a variety of different video/audio formats. Highly recommended!
Apple's Quicktime Player is used to play back MOV video files, it can also play back a variety of other video/audio formats.
Windows Media Player can be used to play back a variet of different video/audio formats, it natively supports the WMV video format.
The DivX codec is required by some media players in order to play back AVI files encoded with the DivX codec.