Free VOIP / SIP phone. Use your PC as a phone. Works with any SIP/ VOIP provider/ phone service. Easy to use interface to transfer, forward calls or set-up conference calls. Free download from http://www.3cx.com

The Abto LLC VoIP H.323 SDK contains a high performance VoIP conferencing client capable of delivering crystal clear sound even for both low and high-bandwidth users and H.323 compatible devices (hardware and software).

Voicent Flex PBX is an easy-to-use software-based PBX system. It supports both analog phone services and VOIP services. Besides PBX features like auto attendant and unlimited call queues, it is extensible through IVR Studio for database access, etc.

Telepati SIP Phone Freeware allows you to make PC-PC phone-phone calls over the Internet. Developed using Research Labs VOIP SIP Phone SDK, this free soft phone brings SIP protocol support for ActiveX. Tested and works with 75% of the providers.

Collanos Phone is a software-only Voice over Internet Protocol (VoIP) phone and a multi-protocol instant messenger supporting Jabber, MSN, ICQ, AOL, and Yahoo! protocol combined in one application.

The free-of-charge VoIP-client allowing Internet users to communicate with each other as by the phone

MiaRec records your VOIP calls. It is compatible with any Voip soft-phone, PBX.MiaRec is a software for recording your conversations in any of Voip protocols (H.323, SIP, MGCP, Cisco SCCP).

Collanos Phone is a software-only Voice over Internet Protocol (VoIP) phone and a multi-protocol instant messenger supporting Jabber, MSN, ICQ, AOL, and Yahoo! protocol combined in one application. It is available on Windows, Mac OSX, and Linux.

SIP Softphone ActiveX - A powerful and highly versatile VoIP SDK. Our SIP Softphone ActiveX provides a powerful and highly versatile solution to add quickly SIP based dial and receive phone calls features in your software applications.

The program Bluesoleil is one of the most popular that exists. It lets you, from your own PC, access and connect to all types of Bluetooth devices. Also, so you can get much more out of that program, they have created BlueSoleil VOIP.