
XBMC ++
The comments for the Revo et al are valid (I have an R6310 sporting Win 7 which I also run a variant of XBMC on called OpenELEC booting off an SD card) but a potentially cheaper (albeit less flexible) option is to get hold of an Apple TV 2, jailbreak it and install XBMC on it. This will handle just about any format you can throw at it (http://xbmc.org).
I'm running two of these streaming media from our QNAP NAS (wired, not wireless before anyone leaps in) and they work like a charm. You do need some basic command line abilities for the initial setup but the process is well documented step by step on various sites.
The bit I like about the ATV2s is that they're relatively cheap (£99 or $99 across the pond), small and have no fans. The fact that they can also stream Netflix as well (recently available in the UK but make sure you have the 4.4.4 software onboard otherwise don't bother) is a bonus. They are limited to 720p and won't output full hi def but as neither of my plasmas are full HD, it doesn't really bother me.