Many web 2.0 startups rely on private betas to initiate the avant garde to their new concept, site, product or community. Once those early adopters have generated enough buzz, more invitations will be available to newcomers. But often, these invites can only be distributed by those already included in the beta. If this strategy is executed well, and your product is interesting enough, invites will be scarce and sought after. For many, this makes an having invite, or being invited, even more desirable.
Perhaps the best known example of this marketing technique was Google's gmail; invites for gmail were, at one point, sold for lots of money on ebay.
So, these days, it's very common to use this method of invitations. But how to obtain such an invitation, if you have no access to early adopters who might have one for you? TechCrunch recently published an interesting article about a really nice initiative: InviteShare. Join the community, donate any invites you might have, and get that other invite you really, really wanted!