Western Digital do a little box that plays all manner of files inc HD stuff. They've done a new one now that will sit on your network too for a bit more money.
The PS3 is fine for most stuff (an update quite a while ago added support for the usual divx, mpeg ones), although it won't do MKV natively. I just convert them with mkv2vob, works quickly (it doesn't transcode, it just remuxes) and it plays the resulting files just fine.
It's a good system, it replaced my old xbox running XBMC for playing this stuff over the network, it's got an excellent iPlayer/youtube interface so if that along with a blu-ray player and an excellent games console appeals then it's easy to justify the extra.