SIP Express Router: Free GPL-ed SIP server (registrar, proxy, redirect)
http://iptel.org/ser/features
De-facto standard
SER: domains, subscribers
Routing language
extension modules
http://iptel.org/download

INVITE: initiates sessions
ACK
CANCEL
BYE
REGISTER
OPTIONS
SIP extension methods:
SUBSCRIBE/NOTIFY/MESSAGE: instant messaging
REFER: call transfer
PRACK: provisional reliable responses
INFO
UPDATE

SIP Phones (User-Agents)
SIP Servers (Registratar, Proxy, Redirect)
SIP PSTN Gateways
Application Servers (Media Servers)