Really, who among us hasn't wanted to run a presumptively unstable operating system? Until today, the delights of beta releases of OS X were reserved to those who signed up for Apple's developer program (and paid the $99 fee to enroll).
That fun and frolic is now open to anyone 18 and over with an Apple ID, via the new OS X Beta Seed program. [As noted on Daring Fireball, before now the OS X Beta Seed program was invitation-only for non-developers. -Ed.] Simply sign in at Apple's beta site, agree to the terms and conditions (including a prohibition on sharing information about the beta versions), and activate the Mac you want to use with the provided Beta Access Utility. Once you're live, the beta builds of OS X will appear in the Mac App Store.
It should go without saying that you need to back up thoroughly before updating to an unreleased Mavericks build; you also should not run beta software on any machine you need for production, time-critical or revenue-generating activities. If you can't live without your Mac, don't dance on the bleeding edge!
[hat tip Engadget]