BeyondCompare is a great utility for comparing folders and files, I mainly use it for diff'ing files during software development, as you can see it also differences pictures.

These are the same Olympus E-P1 raw (.ORF) file processed in Lightroom 2.6 (on the right) and in Lightroom 3 beta 2 (left) to show the subtle difference between the two processing algorithms in a noisy image.

These are default conversions to jpg, no noise removal has been applied in either image.