The plan at Giveaway of the Day is to work with software publishers to offer fully-licensed versions of their software for free for one day as a promotional tool. Publishers gain the word of mouth advertising that comes from happy users acquiring a full license to their products, and Giveaway of the Day gets to be the ones offering the giveaways. And users get free software - so theoretically it's a win-win-win situation.
At the moment, the software titles that have been offered are not familiar to me (with the exception of Frigate3 File Manager), so it seems that at least initially, GAOTD is working out arrangements with smaller software publishers. That's not a knock against them, however - a plan like this can take some time to execute, and already they're offering free commercial titles.
Here's the stated goal at Giveaway of the Day, right from their site:
Yes, we are giving away software, and you can download it from our site, right now and right here and our goal is to give away every good piece of software, sooner or later.
That's not a bad plan!
[Update] There's an interesting discussion going on in the comments. One commenter notes that the software must be installed the same day it is downloaded, or you are required to pay for it to install it. It turns out the terms are set out on the site, and I simply missed them when writing up this post. From what I can tell, it's not a scam, but not quite as free as it seems at first blush. If you're interested in how this site's promotion actually works, check out the comments.