Beta List helps you join private betas ahead of the crowd
I must warn you: never has a single page contained so many hideous top-level domain wanks, and site names ending in the letter 'r'. But, despite the worrying, continuing prevalence of all-lower-case websites, Beta List is rather cool. Of course, for it to actually beat TechCrunch and Mashable to the punch, startup owners will have to actually tell Beta List about their closed beta tests. It has some well-established competition in the shape of The Museum of Modern Betas, too.
You can subscribe to Beta List via RSS, but the newsletter is probably a better option -- that way you can be properly alerted every time a new beta test is listed.