Hashing is a method for reducing large inputs to a smaller fixed size output. When doing forensics, typically cryptographic hashing algorithms like MD5 and SHA-1 are used. These functions have a few properties useful to forensics.
There are literally hundreds of hashing programs out there, but a few related to forensics are:
md5sum - Part of the Coreutils suite, this program is standard on many computers.
md5deep - Computes hashes, recursively if desired, and can compare the results to known values.