You can try to recover the data yourself, or you can send your hard drive off to someone like drivesavers.com (but be prepared, if you go this route, for severe sticker shock).
Professional data recovery can run anywhere from a few hundred dollars to many thousands of dollars, depending on the complexity, size, and format of the data in question.
You can try to recover the data yourself with something like Data Rescue 3, available with a simple Google search.
For maximum recovery possibility, you should stop using your computer immediately. Every second you go on using it increases the chances that the old, deleted data gets overwritten with new data (log files, new video files, etc.). If you wish to have good success in recovering the data, shut down the computer immediately and do not use it again until you are ready to try to recover the data.