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.