Duplicate Files Searcher looks like it might work, I'll try it out later. DupeCheck is spotlight based so it won't work on the NAS, and Tidy Up is $30. I'll try free solutions first, but thanks for the pointer.

Windows is flat out for this, as I need to search inside several .dmg backups and also on my OS X desktop.

Also, thanks for the link to the iusethis.com site, never seen that before. Looks pretty handy for seeing what programs are popular.