The current beta is for 10.9.3 if you fancy testing that for Apple?

Prior to today you would have had to join the Mac Developer Program ($99/year) to test pre-release software.

Join Program (and Download Pre-Release Software for free) / FAQ

Apple has expanded the availability of beta OS X system software to all Mac users, allowing potentially anyone to run the latest pre-release beta builds of the operating system for trials and feedback. Dubbed the OS X Beta Seed Program, this is the first time since the initial release of OS X that Apple has allowed non-developers access to the beta OS builds.

While it may be appealing to many, the beta program is not recommended for primary use Macs or for novice users, as beta software is often buggy and incomplete, offering an experience that is not yet as refined as a public release. Accordingly, average Mac users with a single machine probably shouldn't bother with the OS X Beta Seed program, making this best reserved for curious Mac users who have a spare machine they can run the beta builds on.

Mac users who are interested in this program will need to log into the Beta Seed website with an Apple ID, read and accept a lengthy Terms and Conditions agreement, back up their Macs with Time Machine, and then install an Apple utility to access the beta software downloads. (Source)