It depends on how scratched they are and to which side. If it's the side where the "label" is ... chances of saving them are pretty slim. If it's the other side it depends on how deep the scratches are.
There are several motorized (or hand cranked) machines out that will remove most surface scratches quickly and easily. They can seem a little expensive (depending on your budget) but they are defintely cheaper than replacing DVD's. I have one and absolutely love it.
Another option is plain old ordinary toothpaste. Put a little on the disc ... enough to lightly cover the entire surface, allow to dry. Then using a very soft cloth (a chamoise is best in my experience, but anything that's really soft will work) working from the center to the edge rub the dried toothpaste off in straight lines. Do NOT use a circular motion. And that should also take care of most surface scratches.
I've also heard that car wax or rain-ex works well, but I've personally never tried either so I can't vouche for those methods.
Any that have deeper scratches you might have to search the yellow pages for your area for video stores that offer the service to remove them.