H.323/SIP softphone for MS Windows, Linux, and Mac OS X. Supports G.729i, G.711i, GSMi audio codecs. Fully interoperable with ITSPi and software/hardware manufacturers.Allows for calls to PSTNi as well.

IPv6 support claimed

SIP client With support for Multi-line, Mute, Hold, Transfer, Caller ID, Registration, inbound-outbound proxy, conferencing, Call logging. Integrates with myJabber IM. Custom Branding available.

Supports instant messaging, audio and video conferencing, desktop-sharing and Web page pushing; built-in presence; drag-and-drop user actions; buddy list; media mode migration; rebrandable for OEMs

sipXezPhone, is written using C++, wxWidgets, and the sipXtapi API. Currently both Windows and Linux platforms are supported with MAC OS X a clear posibility.

The sipXphone project, formerly known as Pingtel\'s instant xpressa softphone, is a fully functional SIP softphone that runs on Microsoft Windows and Linux.

SIP Phone for Linux

Looks like a very cool SIP phone. Programmable Java interface.

A SIP multimedia client for VoIPi, Video telephony, Video Conference, Screen Collaboration, Whiteboarding, with Universal IM Platform, Phonebook with Presence, Secure speech; 15-day trial

YateClient is a multiprotocol (H.323,SIP,IAX) VoIPi client. It works on Windows and Linux, and is skinable. YateClient is avaible under GPL free license.

This web-based SIP User agent is a thin-client that uses a Web Browser on a PC platform to provide the services provided by the SIP Multimedia PC Client.

Windows PC-based SIP agent that supports traditional telephony communications while providing additional multimedia communications services such as instant messaging, subscriber provisioned incoming...

Mac and PC

Express Talk is a free SIP softphone for Windows. Fully SIP compliant including multiline support (4 lines), hold facility, DNDi, call transfer, NATi traversal and multiple codecs.

Proven PC client for VoIPi, Video telephony, presence, instant messaging, voice mail, etc. SIP and IMSi environments. Converged mobile-PC services such as video sharing, mobile video call.

open source cross-platform simple SIP softphone, written in java, based on MjSip stack, freely avalible under GPL license.

SIMPLEi client (GPLed)

low-codec video; support for PCs, and pocket/handheld PCs

An enterprise version of MSN Messenger that allows logins to SIP and Exchange 2000 servers (in addition to the MSN server).

Windows XP UA with instant messaging support.

Provides web based SIP telephony functions within the browser. The client supports call hold, reinvites and session timer.

Twinkle is a SIP softphone for Linux

The M2-UA(M2), a SIP Softphone, text messaging feature with SIMPLEi, easy to install, detects firewall type using STUN. Skinnable interface(GUI) with XML, working example using SERi at myeztalk.com

video on Linux IPAQ

PDA softphone

The Icecom Multimedia PBXi is a SIP-based, full featured corporate switch with additional new convergence services.

The Icecom WebClient is a SIP-based communication portal for voice, video and data exchange, IM and conference services.

Easy to use office phone with a USB handset.

adds Hughes SIP stack to SpeakFreely