Mashable Invites is a place to swap invites for "Web 2.0" sites that are currently in private beta mode. Just add your name to an invites list and other Mashable Invites members will drop by and invite you.
"Google and Yahoo have come up with new and very similar plans to respond to the challenge from MySpace and Facebook: They hope to turn their e-mail systems and personalized home page services (iGoogle and MyYahoo) into social networks."