Briefly… Mac OS X Lion (10.7), which you have to purchase from the Mac App Store, but for only $29. You’ll also need a copy of Snow Leopard already installed on your Apple computer ($29, as well). I asked an Apple tech support specialist whether you’ll need to burn a DVD or not to do maintenance on your Mac. He said that Lion will create a partition with a full version of 10.7, and you’ll boot up with that partition to perform maintenance, install (archive, erase, etc.) on your Mac. Apple is moving us into a disc-less world!
But if you want to burn a boot disc of Lion, TUAW has instructions. OS X Daily instructs making a bootable OS X Lion on a USB Flash Drive.
Mac OS X Lion should be available sometime in July 2011 (maybe Tuesday July 26, 2011).