It's still in Beta and I have 5 invites. Just leave a comment with a valid email address...
If you don't know about Joost, here's how it is described on the site:
Joost is a new way of watching TV on the internet. With Joost, you get all the things you love about TV, including a high-quality full-screen picture, hundreds of full-length shows and easy channel-flipping.
The promotional video at the top of this page is fun and well done. Actually, most of the things I'm seeing from Joost are fun and well done.
I wrote about the early Venice Project beta here.