Iso buster used to be good and free, but I think the free version is limited now (not sure).
If it is an important disk, there is a product called plexus that fills the scratches (to a point) with a transparent wax. I have been using it for years, and it has worked on at least half the disks I have tried it on. Even on disks that do not look scratched but will not read (and you suspect errors or bad burn, age etc), it sometimes helps. Its not a cure all but its a cure many. Its primary use is not disks (its used on aircraft windshields, motorcycle windshields, fiberglass body panels etc.), but it does list cd/dvd as a use and it works. So far as I know, your best bet for finding it locally is a motorcycle shop (thats where I got it). I have used a few products that specifically are for cd/dvd that either fill or melt the scratches, and none that I have tried compare.
Beyond that, you have to use abrasives, like brasso, toothpaste, or others that are more agressive, but the plexus is wipe on, wipe off and takes seconds (I have played with doing it like car polish, wipe on, let dry, wipe off, and I think it might work beter, but its still labor free).
Its this stuff