Cue the video of 10.7 walking off into the sunset. Today is Mountain Lion day, and with the arrival of the new operating system comes the departure of its predecessor. Clicking the link for Lion will bring up a "The item you've requested is not currently available" error message in the Mac App Store. Thankfully, Mountain Lion is just one $20 download away for Lion and Snow Leopard users alike.
Apple OS X Mountain Lion
Apple OS X Yosemite