I'm using TwonkyMedia Server (Version 5.1.x) for listing to Music (MP3) and watching my Photos. Small Videos (eg. from Podcasts) are also ok. I didn't try any fancy things like transcoding on the fly. Twonky is a comercial product. They offer a 30 day test version. Downloads can be found here:
http://www.twonkyforum.com/downloads/6.0/ Look for the "Kirkwood" Version.