Not the Windows kind, a standalone unit to play movies; the xbox can only handle xvid which is a dated and horrible format when most pirated shit is in mkv or blue-ray iso's.

So, anyone got any suggestions or things to think about when buying one?

This one looks ok

The only thing you need to look for and be aware of is theres a new copyright being used now called Cinavia. Its rendered the PS3 which used to be the better media player almost useless as it got incoperated into a recent firmware update.

I believe they have also incoperated this into some standalone media players aswell.

I have a Xenta £16 jobbie from eBuyer. It plays from USB or SD. Plays a few formats but not MP4 or MKV (but they appear to be rarely used formats). AVI still seems to be the norm.

A lot of the new films i "get" at the moment seem to be in MKV or MP4 ..

I seem to have a lot of MKVs too. I guess it depends on your source. I tend to convert them and burn them so they can be used in the DVD players.