iTunes has been up to this gig for a while, but now some of the major networks are looking into it as well. Check out the post on TechCrunch for more information.
To me, the most exciting part is that ABC will let you watch episodes of 4 of its most popular shows for free until June 30. So basically that means I have until June 30 to watch the entire 5th season of Alias, since I missed most of it due to the fact that I don't get that station (actually, I don't get most stations. I get two stations and one of them I would never consider watching). To view older episodes, I think you have to start to play the current one and then click "more" on the bottom of the screen to select a previous episode.
Pretty exciting stuff! :)