HashCheck Shell Extension
What is Hash or MD5 checking?
A cryptographic hash function is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string, the (cryptographic) hash value, such that an accidental or intentional change to the data will change the hash value. The data to be encoded is often called the "message", and the hash value is sometimes called the message digest or simply digest.
Basically, when you download something important off the Net, it lets you know that the file has not been corrupted or modified in some nasty way...