Remember when invites to GMail used to be scarce and were sold on eBay for hundreds of dollars (I wonder how the buyers feel now) , well nowadays you don't need to be invited to GMail, but there are hordes of other web services that are invite-only. Now a cool little site, InviteShare, popped up that lets you get invites to invite-only sites (apparently rather quickly), and if you happen to have invites to a particular site, you could share them as well.
Now the concepts is great, and the site is designed very efficiently, but its doomed to fail because of two reasons. Firstly because people are expected to give out invites without any compensation what-so-ever, when GMail were shared, people usually exchanged it for something else, even if it was a postcard from the recipient. And secondly because there are far more people who want invites than there are actual invites, a fraction of which will end up on InviteShare, the majority of visitors won't find any invites for the popular sites.
Still, if you're dying to get into a beta-only site, give InviteShare a spin, and you may get lucky.