So, to be true, this app isn't really free if you want to recover the files the free version finds...BUT you will know if the file(s) you need to recover are there before purchasing.
I recently lost a very important file after changing HDs and upgrading OS, because that particular file was in an obscure path that wasn't included in regular backups. I remembered that I had this file on another iMac that hadn't been turned on in 18 months, however, as luck would have it, its HD was un-mountable when I turned it on!
After a panic, I read about various data recovery software, and installed two after creating a boot disk to get that iMac started. The first one didn't seem to find the file I needed, but Disk Drill did. After a long 15 hours (2TB HD), Disk Drill found over 1TB of data on the un-mountable HD, and it found the file I was looking for!
Sure, I wasn't expecting to pay $95 (w/tax) to recover this file, but in the end I'm glad I did. Keep this in mind: if the files you lost are important to you, you probably won't find a free way to recover all of them. The tech that goes into these recovery apps comes from software engineers that must be paid; so it may hurt a little bit to pay for it, but it's not their fault you lost it in the first place ;-) I think this is a great piece of software, even if the Dr. icon is a bit creepy.