DiskDigger goes well beyond the usual undelete utility that's offered gratis as a leader product, though as of version 0.8 it does that. It will also dig "beneath the file system" (Dmitry's line, which I plan to borrow regularly in the future) to recover data on a sector-by-sector basis from hard drives, thumb drives, etc. (Think of sectors as little boxes containing data that are arranged in tracks/circles on your hard drive.)
DiskDigger couldn't be easier to use. Select a drive, select the types of files to be recovered (jpeg, mp3, documents, etc.) then click next. I actually had a brand new partition-trashed hard drive on hand to test DiskDigger with. To be honest, I wasn't expecting a free program to recognize the hard drive, let alone recover data from it. My bad. It found files--and recovered them.
My only complaint is that if the type of file you're looking to recover using a sector scan (it will undelete any file type) isn't in the DiskDigger's list, it won't find it. Common types that are missing are Outlook personal folders and text files. There should be a "*.*" option or the ability to define file types. Still, many common file types are there and that should cover the majority of home users.
New for version 0.8.1 is the ability to search ISO disk images for missing or existing files. You may now also limit a search to a logical drive (some disks are partitioned into multiple logical drives: C:, D:, E:, etc.) instead scanning the entire physical drive. DiskDigger is a free, easy-to-use recovery program that in many cases works as well as EasyRecovery Professional, Active@ File Recovery, etc. If you need to recover data, your mouse should run, not walk, to the download link.
--Jon L. Jacobi