Free software to recover Camera Pictures

OK, I have been asked to look at a CD with Photos on which contain pictures that do not display. Some of the photos taken will show fine and the icons for all the files are there but some do not show the image when opened. The memory card has been formatted and used again before they realised that not all the pictures had been downloaded ok.

Does anyone know any good free software I could run to see if it will restore these files?

The only thing I can think of is taking it into a camera shop to see if they can sort it out, but obviously free is better :)

Any ideas welcome...