Disk Drill for Mac is an easy-to-use data-recovery tool with a limited free version and more-robust (and more expensive) Pro, Expert, and Enterprise packages. The free Disk Drill Basic can help you protect your computer from future data loss (whether accidental trashing or as the result of more serious hardware problems), while you'll need to pay for at least Disk Drill Pro if you've already lost data that you need to recover.
What sets Disk Drill apart is its slimmed-down interface and almost painfully user-friendly documentation. If you're looking for a data recovery app with a good feature set and some reassuring hand-holding, Disk Drill is a solid choice--and you can't go wrong installing the free version to take advantage of Disk Drill's "Recovery Vault" feature, which can make recovery much faster and more reliable, by tracking a "smart history" of your files (including file names, locations, and activity). When you're recovering a file, Disk Drill's workflow is dead simple: you switch over to "Recover data mode" (from "Prevent data loss" mode), then with a couple of clicks you can choose the drive and then type of scan you want to run, which vary in terms of speed and thoroughness. As with similar apps, Disk Drill also offers S.M.A.R.T. monitoring, backup to disk images, and both quick and deep scan options. Disk Drill can help you recover data on HFS/HFS+, FAT, and NTFS drives, as well as flash drives and memory cards (although Recovery Vault only helps with HFS/HFS+ and FAT).
If you haven't already experienced data loss, you will--and in the meantime, Disk Drill Basic is a great, free way to help ward off the inevitable. If you've already lost data, one of the paid Disk Drill apps can be a good option for getting it back, especially for less technically inclined users.