On the 16th of Feb, Apple announced their new operating system for Mac: Mountain Lion. Taking the Mac community by surprise, the announcement stated that Mountain Lion would be due to release sometime "late summer" which, by looking at previous release history, suggests a mid-August release.
Mountain Lion can be downloaded as a developer preview if you have a Mac developers account. ( $99 a year to enroll, if you were thinking about making one just for the new releases )
Some of the new features due to be included in Mountain Lion include Notes, Reminders, Game Center and Notification Center for Mac ( Such as the ones on iPad and iPhone )
Watch out for a detailed article on All the Features in Mountain Lio on Control Your Mac.