Your friend just messaged you about the coolest new webapp, so you head over to sign up and ... "We're in private beta at the moment." If you strike out with invite-sharing services like Invite Share, the Lifehack.org blog has a few recommendations on getting in without an open invite. One is so straightforward, it's likely overlooked in most cases:
Try to contact someone in the PR department, but anyone with the power to grant invites is good. Then simply offer to review the service if you can get in on the beta now. Have a specific site in mind — if you want to post the review to your blog, be able to mention your readership numbers. Otherwise consider lining up the opportunity to guest post on a larger blog.
From my own and friends' experiences, just writing about a service and its possibilities in comments or forums can end up scoring you an invite. How have you gotten in on a webapp, software program, or other beta list? Share your story in the comments.