NBC has been putting the most recent episodes of several shows, including "Studio 60" and "30 Rock", online for a while, but this is the first time I've seen them make a whole series available. As an added bonus, the online versions of the shows don't have many ads, just a few brief "and now a word from our sponsor" segments in each episode.
It's a great chance to catch up on anything you missed this fall — and with 21 total episodes to choose from, it should keep you busy for a few weeks. (That is, unless you decide to plop down in front of the computer for an 11-hour "Heroes" marathon.) But the deal only lasts until new episodes start airing in January, so you'll have to move fast!