Following up on the Lady Gaga album sale earlier this week, Amazon continues their digital download battle against Apple with the launch of their own Mac app download store. They've got quite a variety of available titles (though, not as much as the Mac App Store of course,) which include big games like Civilization V, productivity suites like Microsoft Office, and a bunch of others. Right now they are offering Airport Mania for free, and your first paid download will be $5 off. We like it.