Windows only 
Have you ever tried to copy a file from a CD or DVD that has been scratched? If so you might have received an error message stating that the file could not be copied because it is corrupt, but there might still be hope if you really need to access the contents.
Copy Cat 2.0 is a free application designed for recovering files from media that has been partially damaged. As long as you can see the file in Windows Explorer this program can attempt to copy it over. What makes this special is that when it comes across an unreadable area it will skip over it and try to copy the remaining portion of the file. Windows Explorer, on the other hand, will stop copying the file once it receives an error.
The great thing about the program is that it works for more than just CD's or DVD's. You can actually use it with hard drives, USB drives, floppy disks, and just about anything else you can store data on.
Get Copy Cat 2.0 for Windows
Note: During installation it will ask if you want to install a toolbar in Internet Explorer. You'll probably want to uncheck the box to make sure it doesn't.
Two more full-featured (except for only working on optical media) alternatives:
IsoBuster free – the premium version adds Blu-Ray and HD DVD support, plus support for non-ISO9660 filesystems including UDF, HFS (Mac), and IFO/VOB (DVD movies)
[isobuster.com]
Recovery Toolbox for CD Free – free version supports CD/DVD/HD-DVD/Blu-Ray
[recoverytoolbox.com]
I was successful in recovering files from CD’s which had been soaked through my open car window a couple of years ago, but I can’t remember if both worked or which worked better.
Thanks for the other tools AshGhost! I actually didn’t realize that IsoBuster was capable of doing that.
thanks
I find unstoppable copier much more usable:
[roadkil.net]
IsoPuzzle [geocities.com] is another one to try.
I’ve found the laser element in a burner will start to be worn out and sometimes the last few discs it makes can be read on it but not other drives. Alignment inside the drives can also affect ability to read as can the type of dye in the disc. Tiayo Yuden has consistenly tested the best for age. It costs a little bit more than others but what is more valuable, a few pennies or the data/time?
Those are the only discs I buy now. They are definitely a higher quality.