I'm surprised that there ISN'T more visible QuickTime support, but that's Apple's bailiwick. Intel and Apple have also begun to crosslicense technology as Apple now gets Intel Video 4.4 support for QuickTime. Of course as far as I can tell, QuickTime is still only available for Macs, 95/98 and NT.
MPG video files are more crossplatform than any of the others. I expect this format is supported under more platforms than any other, however I don't have any information on a LINUX player. If someone has a helpful link, please post it.
As an aside, MainConcept has one of the best movie players I have seen and supports a wide range of formats. It's been the mainstay viewer under OS/2 for a while, and it looks like they've taken an interest in Linux as they are attempting to port their Video Editor over.
Update: 02/13 01:16 by C : I've started a discussion, and someone has already answered my question regarding IV4 and IV5 on Linux. It appears that the only people who can offer support on these codecs is Intel themselves. With their support of Linux in recent times, who knows, this might actually happen. A cordial letter writing campaign might be in order to see if we can get them to port these codecs sooner rather than later?