If you deleted it to your recycle bin you can restore it... if you deleted on mac os x and you have time machine running then you can restore it. If you deleted it and and shut off your computer its highly unlikely any program will be able to restore it (unless you want to spend tons of hours filling in the bits of missing pieces of code because only as small portion of the information is left).
The fact is you delete something to make room on your hard-drive, you can't just delete it from your hard drive and still have it on there... otherwise your hard-drive would have no space left.
My suggestion.. get a 500gig or terabyte external and anything you want to delete just simply cut and paste into that.