Alsoft's DiskWarrior ($99.95) doesn't offer the broad range of hardware diagnostics that TechTool has, nor does it have the array features that Drive Studio offers for tasks such as formatting, resizing partitions, and defragmenting drives. It's designed solely as a drive repair tool. Because of that singular focus, DiskWarrior can generally be considered the most powerful such tool on the market for Macs.
DiskWarrior can repair and rebuild disk directory structures better than most tools; if it cannot repair a disk, DiskWarrior can usually build a replacement directory by examining the files on the disk. In fact, DiskWarrior is so often successful that many technicians are shocked when DiskWarrior cannot repair a disk.
In the process of rebuilding a disk's directory, DiskWarrior optimizes the layout of directory data for improved performance, particularly for tools such as Spotlight and the Finder, which rely heavily on disk directory information. Note that DiskWarrior optimizes only the directory data; it doesn't optimize or defragment the file data on the disk, as Drive Genius and TechTool Pro can.
In its most recent versions, DiskWarrior offers the ability to verify and repair Mac OS X permissions as well as to check for and repair damage to various types of files on a Mac OS X startup drive. It can verify and repair preference files, symlinks, and folder- and file-length limits, all of which can become corrupted and cause serious issues under Mac OS X. It also supports tests of a drive's hardware using S.M.A.R.T. technology.
Although it doesn't provide many bells and whistles, DiskWarrior is often the last-resort tool for problem drives, and more often than not it can repair them successfully. It also has a very clean and easy-to-use interface that includes very good explanations of each feature and step in a repair process. When rebuilding a disk's directory, DiskWarrior offers the option to preview the replacement directory to ensure that it resolves problems, and to copy data from a drive experiencing physical failures. If you're looking solely for a tool for resolving hard drive problems and checking for certain types of file corruption, DiskWarrior is an excellent choice.