[evilpeople]

I've also used Orb. It's free and works very well. You can share live TV feeds, plus music and pictures from your PC. You do need some sort of way to get TV into your PC, but you can usually get them pretty cheap (<$30). Anything you do will work one of two ways - a box that has an input for video and ethernet (much like a slingbox) or a PC with input for video and ethernet. There's really not any other way to do it.

I had really hoped that the invention of IPTV would mean that I could plug AT&T's U-Verse ethernet connection directly into a Media Center PC, but alas, you still need a set-top box and must then run analog inputs to a TV tuner on the PC. Sigh.