Apple just released Mac OS X Lion in the AppStore for $29.99. It comes with a bunch of new features like Mission Control (the new Expose), Launchpad (an application launcher) and updated Mail, Address Book and iCal apps. It also brings the iOS experience to the Mac with no more scrollbars and a new look-and-feel.
If your internet is too slow, you can take your mac to the Apple Store and download Lion there. If you need it on a USB drive, then you will be able to buy it in August for $69.99.
For server users, Lion Server is available as an application for OS X Lion from the Mac AppStore for $49.99 with a host of services like Address Book, iCal, Mail, Podcast, Time Machine, VPN and much more.