WinHasher is a utility to calculate the cryptographic hash of a file. A hash function is an algorithm when applied to a file generate a fixed number (hash) that is unique to that file.
If the file is changed, even by one byte, repeating the hash function will calculate a new hash. Hence, end users can check that downloaded files are not corrupted by comparing the hash of your downloaded file with the hash posted on the software's website (by cutting and pasting the hash from the website into WinHasher).
WinHasher supports several hash algorithms such as MD5 and SHA-1. You can easily generate hashes from Windows Explorer by using the SendTo menu in Windows Explorer (accessed by right clicking on a file).