When you sign up, you are able to join the Mac Giving Tree -- this gives you a few free applications upon sign up. I got Synergy and Enigmo 2 absolutely free. On Christmas Day, you are able to visit the site, sign in, and "unwrap" other presents that show up below the tree. When you refer your friends, you are able to get additional presents to unwrap.
Visit the Mac Giving Tree to get your free applications, and while you're there take a look at the page design. If you Twitter a message starting with @macheist, your tweet will automatically show up on the page ... that's just pretty nifty.