If you bought your Apple after June 6th 2011 with OSX 10.6 Snow Leopard preinstalled, you can request a voucher for the App Store which enables you to download OSX 10.7 Lion for free. Start the process on this website:
http://www.apple.com/macosx/uptodate/
Users from outside the US must replace the .com with their country domain. For example: