Apple announced today that Mac OS X 10.7 Lion will go on sale in July via the Mac App Store (only) for US$29.99. Lion was originally announced in 2010 with the first developer's preview coming out in February. It has more than 250 features, including full-screen applications, multi-touch gestures, Mission Control (essentially Dashboard, Exposé and Spaces rolled into one), Launchpad, the Mac App Store built in, resume, auto save, versioning and more.
Update (1:30 PM): New version of Mail announced. Peer-to-peer Wi-Fi, auto discovery and setup, confirm to send and receive, fully encrypted transfer, snippets, favorites bar, conversations and more. A blow to newcomers, such as Sparrow, for sure.
Update: Lion will be available only in the Mac App Store. 4 GB download, no rebooting, can use it on all personally authorized Macs. $29.99 in July with developer's preview today.
Apple OS X Yosemite
Apple Mac OS X Snow Leopard