siteminder is an application sold by Computer Associates that allows web assets to require authentication before access is given.
Computer Associates SiteMinder is a highly scalable solution that provides best practice identity and access management components for Web single sign-on, authentication, authorization, auditing and administration.
SiteMinder has an API written in Java or C and it is worth checking out the PDFs.
Siteminder is now marketed as "CA Single Sign-On".