CyoHash

CyoHash is a simple shell extension that is used from within Windows Explorer to calculate the MD5 hash, SHA1 hash, or CRC32 checksum of a selected file.

Installers for both 32- and 64-bit machines can be found here, as can the full source code. It's all been made available under a permissive BSD licence, meaning it can be freely used, distributed, and modified for any purpose, provided the copyright notices remain unmodified in the source code.

If you're using CyoHash, please leave a comment!