I've been a hopeless blogger lately but I just came across InviteShare and thought it was worth a mention. I know the Joost invites were doing the rounds a while back and more recently Pownce has had some AIR play (sorry, pun intended!). InviteShare basically takes invitations to the masses, allowing you to register your interest and receive invitations to various private betas. It's also interesting to see what upcoming apps are out there (there are a whole bunch on the site that I hadn't heard of). Worth a look, if you aren't all beta'd out already :)