Data Rescue (
www.prosofteng.com) is the best app I know of for this.
However, you have to be very careful
not to use the computer before you have recovered the files! Everything you do with your computer adds to the probability that the files may be overwritten, making them irrecoverable. Therefore, download Data Rescue onto
another Mac and bring it over via a CD-R or something. Do not write anything on the hard drive, and do not use the computer for anything other than trying the data recovery!