InformationWeek has great article on open source data recovery tools. What type of tools you use will depend on the severity of the situation. You can use live Linux distros designed for recovery like SystemRescueCD or Partedmagic (the latter being more user friendly). Security tools distrubutions like BackTrack can also be helpful; Helix in particular was designed for forensics work. dd is a standard *nix tool for imaging drives, but something like TestDisk can help you repair partition tables for whole disk recovery. Most deletion operations don’t overwrite the data which means you can use file carving to capture the lost files. PhotoRec is able to find files in a number of common formats. Finally, if you’ve got some serious forensic work ahead of you there’s The Sleuth Kit and many other command line tools.
As an addendum, OStatic put together a list of 5 freeware tools for protecting your system.
In your zeal to delete your data, you may have accidentally deleted files that you wanted to keep. Lifehacker has posted this handy list of data recovery tools to help you get those files back.
As you may know, whenever you delete a file, the only thing that changes is the file system. The data of the deleted file is still on the hard drive, but the file system sees the space containing the file as “blank” writable space. Data recovery software typically looks into the directory where the file was stored and scans it, finding any files not listed in the file system.
The program you choose for this task will not only be determined by your OS, but also by the specifics of your recovery needs. Do you need to recover a single file? Many files? A whole hard drive? An unbootable drive? A really scratched optical disk? Specialized tools for all of these needs are available, and this article will help you find the right program for yours.