VLC, arguably (or not) the world's greatest media player - and open source in the bargain - has been ported to the iPhone, according to Wired. If you've never used VLC, you clearly don't download enough video from the web. :-)
The currently beta port, by Zodttd, is a hefty binary by iPhone standards, at 24MB, but it's worth it: you can drag and drop any media file onto a jailbroken iPhone and it should just work. No more tedious conversion using the (quite excellent) Visual Hub.
When Quicktime or Windows Media Player don't open the file (which is reasonably often if, again, you regularly download video from the web), VLC will. It's a fantastic program, and now it's available on your jailbroken iPhone. Get it. Now.