Video player

I don't think there's anything VLC can play which you can't play in Opus. You just need to have the right things installed. (As a bonus, installing them will make them work in just about everything, e.g. Media Center.)

There's a guide here:
[HOW TO: Enable/fix playback of various media formats in Opus)

A VLC plugin is planned, though, for people who don't want to go through that hassle.