I haven't signed up yet but I know http://www5.ballstreams.com/ allows you to watch already played games on demand. You have to join and make a donation. Probably not worth it unless you are looking for a league pass type situation. Their deal is much better.

Tonight's game is blacked out for me on my cable so I decided to purchase ballstreams. It is awesome. I went with 6 months for 59.99. I am watching it through my roku. No buffering so far or any issues. Pretty amazing options. You can choose the home or away broadcast, you can watch a game from the start or in progress, and I believe all games are available on demand after they have been played.