感觉比较出名的就是linphone，x-lite了

软电话Soft Phones (开源)

源代码允许下载，修改

名字	描述
ActXPhone	An ActiveX-Control SIP Softphone based on the Microsoft Real Time Communications (RTC) API.http://www.pernau.at/kd/voip/ActXPhone/. vb写的
Ekiga	SIP, H.323 audio and video softphone for various linux, solaris, windows, and various unix systems. Formerly GnomeMeeting
IAXComm	IAX2 softphone for Windows, Linux & MacOSX using IAXClient.
JsPhone	Java SIP 软电话与视频支持
Kiax	IAX softphone for Linux using QT (runs well also on GNOME), ported to Windows (QT4).
KPhone	SIP 协议  用于KDE.
JPhone	Juphoon open source business softphone SDK support Linux, Windows, Threax and more OS.
Linphone	http://www.linphone.org/index.php/eng/download 视频软电话（ Linux and Windows ）使用 osip 库.
MozPhone	IAX addon. Text and URL support, and manager interface. http://moziax.mozdev.org>MozIAX
OpenPhone	OpenH323. （这个很老了，简单的封装了openh323的函数而已）
Peers	Minimalist SIP softphone written in Java. Tested on Linux and Windows. ITU G.711.
QuteCom	Linux, MacOSX, and Windows software with voice, video, and instant messaging - old name: OpenWengo
SFLphone	Multiplatform softphone from Savoir-faire Linux, SIP, IAX.
Shtoom	SIP softphone written in Python. Runs on Windows, Mac, Linux.
sipXphone	SIP softphone from SIPfoundry, previously known as the Pingtel phone.基于java，界面 －》
sipXezPhone	"sipX easy phone", a SIP softphone bassed on sipXtapi.
Telephone	SIP softphone for Mac OS X written in Objective-C/Cocoa.
Twinkle	SIP softphone for Linux.
wxCommunicator	SIP Windows, C++, based on sipXtapi.
YakaPhone	Simple, skinnable IAX/IAX2 and SIP softphone from YakaSoftware.

• minisip 迷你sip电话，支持很多平台linux，windows，Pocket PC，需要gtk支持。

http://www.minisip.org/index.html

软电话Soft Phones (免费，不开源)

• x-lite:

http://www.counterpath.com特点：这软体电话可说是最经典的软体电话。

• Gizmo http://gizmoproject.com/intl/zh-Hant/   号称可以干掉skype的软件

• IAXOCX -a free IAX2 ActiveX Control based on iaxclient by ipxchina.
• 3CX VOIP Phone for Windows Free SIP / VOIP phone for Windows
• BOL2000 时曦科技开发的sip软电话 在国内比较流行

• Cubix: IAX and SIP Protocol support for Windows.
• DIAX: IAX protocol softphone, iax1/2 vb client
• Diamondcard.us Softphone Worlds first IAX softphone with a flash interface. Private label available. Easy install.
• Doddle: SIP Web-Driven Doddle softphone (applet webphone).
• ENUM RTC Softphone Windows Softphone with ENUM support
• Express Talk Softphone - Express Talk SIP Softphone for Windows (free)
• http://www.globaliptel.com X-TAPI lite/Pro: TAPI-based SIP Softphone for MS Windows. 9,95 EUR. Also has a free soft phone.
• iaxLite: IAX softphone with AU-100 USB phone set support.
• Idefisk: IAX2 and SIP freeware softphone for the Windows / Linux / MacOS platform with a focus on small size, small memory footprint and high quality.
• JackenIAX:IAX2 Softphone for Macintosh Mac OS X 10.4.
• MCE SIP Phone A SIP softphone for Windows XP
• MizuPhone A new fully featured and costumizable softphone with G729 codec
• PacPhone Secure Nat-Aware H.323 PBX softphone, Native Nat GnuGK, STUN/UPnP(symmetric) AES256 Encryption,PBX,VoiceMail,USB phones
• Phoner Windows SIP softphone (also supports ISDN cards)
• PhonerLite Windows SIP softphone
• Raketu Voice, DialOut, All messengers, Media Player, SMS, RSS, IPTV/VOD, (Communication, Information, Entertainment)
• sipc: SIP ua for WIndows, Solaris & Linux. Includes support for Video & Whiteboard
• sipLite SIP softphone with G729 codec.
• SIPPS Free: Free lite version of SIPPS
• SJphone: SIP, H.323. Windows, Linux, Mac OS X, Pocket PC.
• TekPhone: VoIP (SIP) softphone for Windows.
• Telesis XPhone: VoIP softphone for Telesis IP PBX systems. The applied VoIP protocol is Telesis xSIP (eXtended SIP) protocol
• Tpad - The Global VoIP Network Free PC VoIP SIP Softphone / Dialler for Windows 98/ME/2000/VISTA, SIP softphone supports STUN (stun.tpad.com) with choice of G711 / G729 codec.
• Voix Phone Free IAX/AX2 Softphone for Windows, Linux & MacOSX.
• Zoiper IAX / IAX2 and SIP softphone, g729 (optional), customization packages, various features; for Windows, Mac OS and Linux. Solaris 10/SPARC beta version available from here.
• PCBest Network: An Free SIP ActiveX Control to download. It can be used in IE or Other Windows application. It support multipul audio codecs and NAT, STUN.
• Zultys phones: SIP softphone for Linux - basic free, extra features chargeable
• VoiceClearly First JavaFX gTalk Client - Develop custom gTalk integration using Java API

参考：http://www.voipinfo.org/wiki/view/VOIP+Phones?navmenu