I’m always needing to check the integrity of ISOs and other large downloads; I use md5sum on Linux – and now a new tool on Windows.
digestIT 2004 is a free (beer not speech) tool for calculating the md5 and sha-1 checksums of files.
There is also an interesting project – the Antares Project – which seems to move the integrity checking game a lot further. FIPS-140-2 certification too. That’s interesting.