Yup, that site is legitimate. You may be required to install a program called Steam in order to play the games. As long as you download Steam from the Valve website, it's safe to install.
However, the download you're looking at has the Windows logo (the 4 squares) and I don't see the Mac/OS X one, which is usually an Apple. Make sure the games you're planning to install will work on your Mac before purchasing them.
~Lyssa