I use my PS3.
It will play...
http://a8t8.spaces.live.com/blog/cns...13E8!188.entry
For streaming from the PC, I use
PS3 Media Server (as mentioned above by Graham). You can also of course use any other UPnP Media Server, such as tversity or WMP11/12, but I prefer PS3 Media Server. It will stream everything the PS3 can natively handle, plus transcode/re-mux-on-the-fly & stream anything the PS3 can't natively handle.
There are easy ways to play .mkv files on the PS3 if using a server which doesn't transcode stuff (e.g. WMP): Use something such as mkv2vob or tsMuxeR (both free) to re-mux your .mkv files into a PS3 compatible container (e.g. .vob), without losing any quality (as no transcoding takes place, the H.264 content is simply repackaged into a container the PS3 can handle).