Apple Quicktime
http://www.apple.com/quicktime/download/
VLC Media Player
http://www.videolan.org/vlc/
K-Lite Codec Pack (to play darn near every video codec in the world)
http://www.free-codecs.com/download/k_lite_codec_pack.htm
Windows Media Player
http://www.microsoft.com/windows/windowsmedia/download/AllDownloads.aspx?displang=en&qstechnology=
Real Media Player
http://www.real.com/realplayer