CGSecurity's...

PhotoRec & TestDisk

...Open Source utilities can help you analyze and (sometimes) recover files from a variety of disk formats including flash cards.

CGSecurity provides executable binaries for Microsoft Windows, Linux, and Mac OS X. I tried the version for Windows XP on a known bad Lexar 1GB SD memory card. You can see in the image that TestDisk identified a number of sector errors on the card.