S/MIME Freeware Library (SFL)
- An open source implementation of the IETF S/MIME v3 RFC 3369 Cryptographic Message Syntax (CMS) and RFC 2634 Enhanced Security Services (ESS) specifications.
- http://www.digitalnet.com/knowledge/sfl_home.htm

Secure MIME (S/MIME)
- Comparison between OpenPGP and S/MIME, plus the RFC's that relate to the Secure MIME standard.
- http://www.securemime.com

SMIME.org
- Promotes email encryption usage, by providing help and references to email encryption products and standards.
- http://www.smime.org/

S/MIME Working Group
- The resource site of IETF S/MIME working group, with links to draft specifications. (Internet Mail Consortium)
- http://www.imc.org/ietf-smime/