Authentic is a full-featured Liberty-compliant identity provider aiming to address a broad range of needs, from simple to complex setups. (GNU GPL)
mod_mellon! is a free (GPL) SAML 2.0 authentication module for Apache 2.
SimpleSAMLphp is a free (GPL) implementation of the SAML2 protocol in PHP. We often advocate it instead of our own PHP5 binding because it's easier to deploy (non binary dependency).