Data Rescue II and FileSalvage are both solid apps. Both are pricey ($80-$100 if I recall correctly), but can be lifesavers if you really need them. I believe that both have a free demo mode that will let you see if you might be able to recover your files.
The most important thing to do is to immediately stop using the computer. Every second it's writing data to the disc, you increase the chances of it writing over your deleted data.