TVSquad

Mike Garcen of Missing Remote has written up a detailed comparison of three programs that leverage your PC to let you watch live TV from any internet-connected PC.

Orb, WebGuide4 and SageTV Placeshifter are all programs that let you access your home computer to watch and record live TV over the internet. SageTV Placeshifter is designed to work with SageTV, and has an interface that looks just like the desktop application. WebGuide4 works with Windows Media Center, and Orb works with pretty much anything.
That's probably why Orb came out on top in most of the categories where Garcen made comparisons. It's free, it's powerful, and it doesn't restrict you to using one particular PC-based PVR program.

That said, if you have Windows Media Center, WebGuide4 would be a great addition. The free version enables remote scheduling of recordings, and for $18 you can pick up a license that will let you stream content. And keep in mind, this program is so cool that the guys at Media Portal felt the need to rip it off in order to get some slick graphics for their web interface.

And for $30, SageTV Placeshifter gives SageTV viewers an experience that's practically identical to their home viewing experience.

Orb and WebGuide4 also have mobile interfaces.