Mac OS X: any of the common video players will handle these files, including the stock QuickTime Player. VLC or Mplayer are other options, able to play a bewildering variety of formats with no further codec installations required.
*BSD or Linux: you already know how to play these. ^_^ VLC is one of many choices.
Windows: there are various options, perhaps simplest being the QuickTime Player. If you have a fast connection, the standalone installer may be simplest, especially if you're wanting to perform more than one installation. There's also VLC and Mplayer, which can both handle many other formats as well. Media Player Classic may work, but with image corruption for the first few seconds.