About a month ago, I discovered (following someone's Tweet actually) the MacHeist site. Basically they have secret agent missions with puzzles you have to solve. At the end of each "mission" you get "loot" which is normally some piece of shareware Mac software, with a license code included. The coolest part isn't just getting the software, but discovering new software. For example, one of the "loots" was Fresh, which I'm using a bunch now. There's 2 types of missions - Nanomissions which are pretty quick puzzles, and full on Missions which take quite a bit longer (the last one included solving semaphores, and overlaying a map onto another map). One of the Nanomissions involved solving sentences that said things like "52 C in a D" (52 cards in a deck), and then finding the item in a messy garage (which was almost harder sometimes).
Once all the missions and such are over, they do a Bundle Reveal which is the ability to purchase a Mac shareware software bundle for extremely cheap. The bundle just revealed does contain some stuff I'm not interested in, but I ended up buying it anyways since the software I was interested in, was worth the $39 price ($31 for me with completed mission discounts). This bundle includes an Ebay auction writing/searching app, several graphic editors (one which is a smaller version of Photoshop), games, a video editor, several others, and some which have yet to be unlocked, such as a WYSIWYG HTML editor and a todo list app (for Getting Things Done).
The puzzles are just really sweet, and I wish I hadn't discovered the site so late. Hopefully there will be more missions and such.
More info on the bundle at the Macheist Home Page.
(edited to remove referral link, since I got my extra software)