CheckSum

checksum a point-and-click sha1 and md5 hashing application for windows.SHA1 and MD5 hashes are used to verify that a file or group of files has not changed.

Simple as that. This is useful, even crucial, in all kinds of situations where data integrity is important. For instance, these days, it's not uncommon to find MD5 hashes (and less rarely now, SHA1 hashes) published alongside downloads, even Windows downloads. This hash, when used, ensures that the file you downloaded is exactly the same file the author uploaded, and hasn't been tampered with in any way, trojan added, etc.; even the slightest change in the data produces a wildly different hash.