SIPML5

After the world's first SIP video clients for Android and iOS (early 2009), Doubango Telecom open sourced the SIPML5 Project. SIPML5 is the world's first open source HTML5 SIP client entirely written in javascript for integration in social networks (FaceBook, Twitter, Google+), online games, e-commerce websites... No extension, plugin or gateway is needed. The media stack rely on WebRTC project. The client can be used to connect to any SIP or IMS network from your preferred browser to make and receive audio/video calls and instant messages.
Short but not exhaustive list of supported features:

Audio / Video call
Instant messaging
Presence
Call Hold / Resume
Explicit Call transfer
Multi-line and multi-account
Dual-tone multi-frequency signaling (DTMF) using SIP INFO
Click-to-Call
For more information: http://www.sipml5.org