SIP is arguably the biggest thing since e-mail. It defines an open protocol for real-time communications. Forget phone numbers. Forget AOL Instant Messaging. Imagine all forms of real-time communications federated under a single, open protocol: Instant Text Messaging, Voice, Video Conferencing. Read more.
All this time i've been looking for free software to create SIP infrastructures by googling for "open-source SIP software" and coming out mostly dissatisfied until i finally thought to look at my Xten client connection logs to see what EarthLink was using, and googling it: SIP EXPress router from iptel.org.
We've got open protocols. We've got free, open-sourced GPL software. Let the SIP insurrection begin.