Product Name: Windows Messenger
Product URL: http://www.microsoft.com/windows/messenger/
Vendor: Microsoft
Supported Protocols: SIP
Platform: Windows
Description:

A well-know SIP softphone from Microsoft. There are different versions of the software, not all of them support SIP. This is unfortunately confusing. The most standard compliant seems to be version 4.7. The phone is easy to use, but there are several violations of specifications. It supports audio, video, instant messaging, white board.

The client sends BYE instead of CANCEL. Microsoft uses a proprietary extensions of the presence document, some other software vendors implemented the extensions as well. The realm of digest credentials must be same as the hostname of the server, otherwise the client refuses to authenticate.

New versions of Windows Messenger use INVITE to establish a session before an instant message is sent and thus are incompatible with other user agents, even with older versions of Windows Messenger. It does not support any NAT traversal techniques but it can be used from behind NATs with some aid from SIP server in the public internet because it implements symmetric media and signalling.

Product Name: kphone
Product URL: http://www.wirlab.net/kphone/
Vendor: Wirlab
Supported Protocols: SIP
Platform: Unix/Linux
Description:

A user agent for linux. Versions up to 4.0 were linked with KDE, starting from 4.0 kphone does not require KDE libraries anymore and uses qt only. The phone supports presence and instant messaging based on SIMPLE standards. Video conversation is supported using vic. The phone is not very stable yet and crashes from time to time. It supports G.711, GSM, and iLBC codecs.

Product Name: Linphone
Product URL: http://www.linphone.org/
Vendor: Simon Morlat
Supported Protocols: SIP
Platform: Unix/Linux
Description:

A simple SIP user agent for linux, uses the GNOME frameworks. The phone is unstable and crashes from time to time.

Product Name: X-Pro, X-Lite
Product URL: http://www.xten.com/
Vendor: Xten Networks
Supported Protocols: SIP
Platform: N/A
Description:

A good softphone with many interesting features. It has sophisticated NAT detection mechanism. SIP support is good, there seem to be no interoperability issues (as far as we have tested). X-lite is a free version, X-Pro must be purchased. The softphone is available also for WindowsCE based PDAs.