Plex (http://www.plexapp.com/) is the simplest media server out there right now. You run "Plex Media Server" on any device you want to stream media from, and run the actual "Plex" application on the computer connected to the TV. The server and actual application can run on the same computer. Plex can even stream from the server to a mobile device (iPad, etc...) with their Plex app.
Additionally, Plex has lots of add-ons that are incredibly easy to install so you can watch Netflix, Amazon Video, Hulu Plus, The Daily Show, and The Colbert Report (among lots of other services) right from Plex. Quality is awesome and the software looks great and is super responsive, even on an older computer.
Just want to throw in something I haven't had much luck with. Its called Orb - basically sets up a web server. The trouble I had was (I was using the Wii to view) I couldn't get it full screen and it was very slow/choppy. You might have better luck though.
www.orb.com – Ciaran – 2009-08-01T12:22:55.530