Starting today through June 30th, ABC is experimenting with streaming some of their primetime shows on the web, for free. Now, you will have to watch ads (though you do that when you watch these shows on TV, so that isn't a deal breaker for me) and you'll have to watch them on your computer.
The good news for Mac users? ABC is streaming these shows via Flash video, so we can actually watch them on our Macs (as long as you have Flash 8 installed, of course).
Update: I failed to mention that this is USA only.
Thanks, Dave Zatz.