Apple unveiled its next revision of OS X, christened Yosemite, at its Worldwide Developer Conference in San Francisco on Monday. Developers will receive the new version today, and everyone else will be upgraded in the fall. The good news? It's free. And you'll definitely want it: More than a pretty makeover, OS X Yosemite will offer better notifications, the new Spotlight search app, a revamped Mail app and more. Read on to learn about the biggest productivity improvements.