OS X Mavericks Developer Release 3 is available for download
OS X Mavericks was unveiled last month at Apple's Worldwide Developer Conference (WWDC) in San Francisco. Like iOS 7, it's currently in development and expected to be released this fall.
Just a reminder: pre-release software like this is intended for registered Mac developers only. Please don't make problems for yourself by putting this on a Mac that has important data or is otherwise mission-critical. Make sure to back up before you install it, and back up often while you're using it.