From the "too good to be true" Department comes TV Links, a site that aggregates TV shows available for viewing online. After clicking on a few links to shows an finding paydirt, I was feeling encouraged. I decided to click on a few more before declaring it "worth checking out." The final tally was 6 out of 10 links lead to viewable shows. Actually 5.5 since one show seemed to start partway through.
So I'd say it's probably worth checking out but don't get your expectations up too high. Selection is a bit scattershot, but not bad. Plenty of cartoons and a few popular shows like Battlestar Galactica and 24, but you may have to cross your fingers a lot.
It's a ridiculous shame that sites like this even need to exist.