Hour 17 covers access control based on certificates and parameters of an SSL connection.
A great number of modules provide access control against various databases, NT domains, Pluggable Authentication Modules (PAMs), and so on. Most of the modules run on Apache 1.3, but they are being ported to work with Apache 2.0. Hour 24, "Additional Apache Modules and Projects," mentions some of them, such as mod_auth_ldap.
Digest authentication is described in RFC 2617. The MD5 algorithm is described in RFC 1321. You can download both RFCs at http://www.rfc-editor.org.