I have already tried a few UPNP servers, the best being mediatomb so far. However, I recently bought a new television which is UPNP/DLNA compatible and I had to look for something more video friendly. I found PS3MediaServer which is able to stream and transcode music, photos and video by using the well known mencoder/mplayer software.
PS3MediaServer is a java application which is really plug'n'play, or run and enjoy: it just basically worked when I launched it. No tedious configuration, no installation needed, and it just works. Great work!