QuickSilverScreen.com
A pretty handy site, has links to a lot of seasons of popular TV shows, and most of them are hosted on YouTube and Google Video, making them pretty reliable.
Some are hosted on horrible sites like LiveDigital, and it can be a bit annoying to hit one of those halfway into a good season of some show, but considering that it's free, I find it pretty easy to put up with.
Give it a look!