In My Opinion: XBMC!

I figued it was time to pimp something I use daily: Xbox Media Center (or centre for you mispelling brits).

Xbox Media Center is an applicattion for playing nearly every type of media on a modded xbox (360 version should be out in about 2 months). And I do mean nearly every type, Divx, Xvid, Mkv, Mp4, QT, WMV, RAM, 3vix, VP4(&3), Theora, ASF/x, and way too many more to list. It also has full network connectivity, meaning it can use samba shares, and well as xbmsp. Not to mention it accepts third party python scrpits so the possibilites are just about endless.

If you don't have a modded xbox this is reason alone enough to concider modding yours or buying a modded one tho the latter is much more pricey. With "no-solider" chips becoming so
inexpensive that a better option in my opinion, in fact you can buy an X3 chip (no-solder) for a bit over 60$ bux, and that a damn solid chip (as well as an exteremely long sentence)

Xbmc also just recently got a facelift with a skin called MC360 by Team Blackbolt. This skin will give XBMC the look and feel of the 306 gui, with the same "blade" system that make it so easy to navigate. It the first that the hearlds XBMC2.0. The forthcoming version of XBMC is reported to extetend it now impressive feature list.

Here's a video to give you more of an idea how it looks.

That's not to say the oniginal skin (PMIII) isn't useful of functional, in fact you'll probbabally find yourself switching between the two.

And there are a slew of XBMC skins out there, though most will not function properly with the new (T3ch) xbmc build. Another great one to check out would be Project Dark Matter.