PS3 Media Server is a Java-based DLNA-compliant UPnP media server that's designed to transcode and stream media files across the network to your PS3.
The program will quickly stream all the formats that the PS3 natively supports, including images (JPG, PNG, GIF, TIFF), music (MP3) and plenty of videos (AVI, MP4, TS, M2TS, MPEG). Other formats - MKV, FLV or OGM, say - will be transcoded in real time, courtesy of the bundled Mplayer and FFmpeg packages. (Or, at least, that's the plan. You'll need relatively powerful hardware for this to run smoothly.)
There's support for streaming all kinds of other media, too: picture-based feeds like Flickr and Picasaweb, internet TV and radio, audio and video podcasts, and more.
And with clients for Windows, Linux and the Mac, along with basic Xbox 360 support, PS3 Media Server should be able to adapt to just about any network setup to deliver the results you need.
It's occasionally complex, and a little demanding of your hardware, but once you've got PS3 Media Server set up then it's an excellent way to stream media files around your home