Command line tools for quickly comparing files and directories.
Ever need to know if a set of files was transfered correctly but are unable to use rsync? Maybe checksumming both sides will take too long, maybe the filesystem was remote or removable. Use packingslip!
Packingslip allows you to create a packing slip just as in the world of shipping with which you can verify that the contents are all present and intact. As a secondary benefit this allows you to verify integrity without necessarily checksumming both sides -- since the checksums were recorded during the packing slip creation.