Devs and other eager Mac users should keep an eye out for the latest build of OS X Yosemite. Beta 7 is rolling out steadily to users right now. Just check the Mac App Store on a machine running an earlier version to see if you're ready to upgrade.
If you're hankering to give this a shot, you can apply for the beta program over here. It's still early, so we haven't really dug around too much, but we're eager to hear how your early Yosemite experience are going. Shout out in the comments!