mpg/mpeg ... what you need for iTunes is mpeg 4 or H.264 which is a variation of mpeg 4. Unfortunately there's no easy way of seeing the difference between mpeg 1, 2 or 4. So it's a little like trial & error. Alternatively, you can convert them if you have QuickTime Pro. There's also some 3rd party programs that will do it.
As for general viewing ... we always recommend VLC - (click on link)
_ (choose nearest mirror site) because it has all the plug-ins built into it & can play most things ... here's what it can play -> VLC - Features
which means it can play most audio & video formats, is good for audio & video streaming etc.