VLC's latest version: 81,297,899 downloads and rising
As cross-platform, plays-every-type-of-media-file, and-for-free players go, it's tops in a lot of people's bookmarks. It's one of those toolkit apps that we have ready to go for the right occasion. You know, those moments when a friend gets a new Macbook and wants to play AVI (or MPEG, MOV, MP4, and many more) files. They ask you, and you fire back "VLC" without hesitation.
It's tough to get this type of mass following and community around an application, but VideoLAN has done it.
With really easy installs on Windows, Mac, and all of the popular flavors of Linux, there's no excuse not to give this a try if you haven't yet.
The major knock on VLC has been its bland interface, which obviously isn't their main focus with this app. But with skins you can liven it up a bit. It even works with Winamp skins. Remember Winamp? Of course you do, heck, some of us still use it (but refuse to admit it).
Give it a shot, put it in your toolkit. Pretty or not, it gets the job done.