Blink is simply the best SIP client . You can use it with any SIP provider or Blink provided fully-featured SIP service. A state of the art, easy to use SIP client. Now available for MacOSX, coming soon for Windows and Linux.

Voice Operator Panel (VOP) is a professional SIP softphone for operators and receptionists with Outlook/LDAP/MSN/CRM integration, built-in web browser and e-mailer.
VOP can be used as a standalone softphone or combined with any analog or VoIP slave phone to handle calls.
VOP offers a fully customizable workspace with the same skins than the Microsoft Office Suite.
VOP provides full extension status monitoring (ringing, busy, available) with calls details and call interception.
VOP is ideal for IP Centrex operators and IP PBX installers who want to offer to their customers an advanced software switchboard and attendant console.

The Switchvox Fire Dialer extension works with the Firefox web browser and the Switchvox PBX to allow you to click phone numbers on any web page and have that number ring your extension. 

Features:

Allows dialing numbers from any web page
Imports your Rapid Dial list to allow calling from your browser
Has a highlight feature to make finding phone number easy
Conviently keeps track of your dialed numbers

WILDIX is a FREE IAX  Softphone it's windows based and support:
* All Wildix USB phones ( K1000, Perfectone UP-90, BeyondTel SX-06 )
* All Asterisk PBX
* IAX2 protocol

Features:

* Easy user interface
* Minimum system requirements with maximum standard softphone features
* Blind Transfer and Assisted Transfer are completely supported
* Hold and Flash button
* Supports Up to 6 lines
* Supports CallerID
* Gives superb voice quality on all version of Windows