Though it's a bit pricier than much of the competition (except for OnTrack's pricey EasyRecovery Professional), R-Studio Data Recovery ($80) is as effective as anything out there at retrieving disk data--and it's more versatile than most. Where most sector-based recovery solutions handle only FAT and NTFS partitions, R-Studio Data Recovery also works with Linux and Mac OS file systems. It also copies and images drives, as well as offers several features for dealing with RAID. This is a big-dog tool, and it's especially handy if you're working in a multi-platform environment.
R-Studio is easy to use (once you're acclimated), and has much the appearance of a disk defragmenter: There's a list of drives in the left-hand tree and a sector-block map on the right. You can see each block change color as it's scanned, and again if something is found--a nice bit of visual feedback. The interface is a bit technical in its language and options, but so is the task. The only tricky part is realizing that everything is listed in tabs and on a directory tree. If might be nice to get a dialog that says "go here" upon the completion of a scan, but once you're used to looking to the tree for all things, navigation is a breeze.
Like its competition, R-Studio will do simple undeletes. Every scan is sector-based, however, so it's not the optimal tool for that particular job. Use the free Recuva (or something similar) if all you want to do is quickly scan the file table and undelete a file.
R-Studio Data Recovery comes in a number of different flavors besides the plain $80 version I tested. For example, there's a Network version for recovering across a network, and an Emergency version that runs off of a boot disk. With the $80 version you'll need to attach the damaged drive to a working system where you've installed R-Studio Data Recovery.
R-Studio Data Recovery is as good as it gets in software-based, sector-by-sector data recovery. If the data is not damaged or overwritten, R-Studio Data Recovery should find it. Because of its versatility and reliability, it's my go-to program. I also choose this utility when a drive is in danger of expiring and I may only get one shot.