Kamailio is a very fast and flexible SIP (RFC3261)
proxy server. Written entirely in C, kamailio can handle thousands calls
per second even on low-budget hardware. A C Shell like scripting language
provides full control over the server's behaviour. It's modular
architecture allows only required functionality to be loaded.
Currently the following modules are available: digest authentication,
CPL scripts, instant messaging, MySQL and UNIXODBC support, a presence agent,
radius authentication, record routing, an SMS gateway, a jabber gateway, a 
transaction and dialog module, OSP module, statistics support, 
registrar and user location, SIMPLE Presence, Perl programming interface,
SNMP and Java SIP Servlet.

WWW: http://www.kamailio.org/