e-Lock PKI
A certificate issuance system that is integrated with CryptoAPI. [Free] (e-Lock)
Entrust/PKI
A certificate issuance system for Windows NT, HP-UX, Solaris, and AIX. (Entrust)
Netscape Certificate Server
Web-based certicate server for HP-UX, Windows NT, and Solaris. (Netscape)
pyCA
Python software for running a certificate authority on UNIX. [Free / Open Source] (Michael Ströder)
UniCERT
A componentized certificate authority system for Windows NT. It is capable of issuing SSL, S/MIME, IPSEC, SET, custom certificates, the delivery method can be e-mail, web, face-to-face. (Baltimore)