QuteCom Previously known as WengoPhone, Qutecom is a free, SIP compatible VoIP softphone initially developed by Wengo. QuteCom supports a range of VoIP codecs including G.729, G.711, iLBC, G.722 (wideband) and Speex. H.263 for video is also supported.
XLite from Counterpath A very popular, free SIP softphone supporting a range of codecs and also offering great support for desktop business video conferencing.
ZoIPer Features support for both SIP and IAX, and includes free and paid versions of their software.
Firefly by FreshTel Free software supporting SIP and IAX, as well as a range of codecs.
DIAX Another free, open source softphone supporting both SIP and IAX. This one aint pretty, but has amasses a loyal following amongst hardcore, technical types due to the wide range of tweaking and configuration options it affords.
ExpressTalk Offers STUN and SIP support.
Damaka Self described “peer to peer” SIP softphone.
AdoreSoftphone Supports SIP RFC 3261 compliant stack.
MiniPAX Supports g729, g723.1, speex, gsm, and g711.
MizuPhone Has extra features like HD video, Remote Desktop over SIP and UltraWideband codec.
FlashPhone Unique Adobe Air , web based SIP softphone.
FaramPhone SIP softphone which utilizes the NGN, 3G network standards to make it be the potential communication terminal in both NGN & 3G networks. Users can use the softphone to make Audio, Audio & Video calls and conference among these networks.
Mirial Softphone Supports SIP and HD Quality, 720P video transmission.
YakaPhone Simple, skinnable IAX/IAX2 and SIP softphone from YakaSoftware.
WXCommunicator SIP Windows, C++, based on sipXtapi.
Twinkle SIP softphone for Linux users.
Ekiga Ekiga (formely known as GnomeMeeting) is an open source VoIP and video conferencing application for GNOME. Ekiga uses both the H.323 and SIP protocols. It supports many audio and video codecs, and is interoperable with other SIP compliant software and also with Microsoft NetMeeting.
IAXComm IAX softphone for Windows, Linux & Mac OS X
SJLabs SJPhone SJPhone is a softphone which allows you to speak over Internet using any desktops, notebooks, PDAs, stand-alone IP phones, and even any traditional landline or mobile phones. It supports both SIP and H.323 industry open standards and is fully interoperable with most major Internet Telephony Service Providers (ITSP) and software and hardware manufacturers.
Phoner Phoner is a free but not opensource Windows SIP softphone. It also supports ISDN cards using a CAPI interface.