If you're feeling adventurous and can act fast, you can now sign up for access to Apple's Mac OS X Yosemite beta.
Yosemite is a major update over the current OS X Mavericks, with a new look and improvements to built-in apps such as Mail and Safari. It also has some deeper hooks into the iPhone and iPad, letting users pick up on one machine from where they left off on another, and answer phone calls through their laptops. (Those features won't hit until iOS 8 launches this fall.)
Keep in mind that as a beta, bugs and glitches will be inevitable. Apple suggests installing Yosemite on a secondary Mac, and only after backing up its contents.