I downloaded AppsGoneFree for this thing. I have had it long, but I also went back and looked at past day sales. There have been a few interesting apps, but I haven't added any from the site yet. My devices are a bit full at the moment, so I need to make some priority decisions. Plus, many of the interesting apps I saw were purely entertainment, and my life is really busy right now - adding distractions is not a positive move. I will probably continue to monitor this app and see if something really great comes along.
Although you didn't mention it, some participants out there may have Amazon Prime. If you do, there is a free app of the day as part of your subscription ... I've gotten several great apps for my KindleFire through these deals.