Skype™, Google Talk™, and Vonage™ are Internet telephony service providers with their own softphones, but these three major providers are not interoperable, and you can't place a direct call between them.
Portsip is known for making Software Development Kits or SDKs, which is the core of the softphone. Pangolin™ has a good interface and allows for personalization. It also offers video conferencing feature and IM interface, which gives you more options to communicate with your contacts.
Pangolin’s free download, however, is only available for Windows 2000/XP/Vista/7™, Windows Mobile 5/6™, and Nokia S60 3rd FP2™. So far this is the easiest program to configure of all I’ve tried. After installation, you only need to enter your OneSuite VoIP username (OneSuite account name-voip.onesuite.com), password and proxy server (domain), and click “Login.” Then you’re ready to make calls.
Username: OneSuite account name-voip.onesuite.com
Password: Softphone 8-digit password
Proxy Server: voip.onesuite.com
You should be able to see the status “Logged in” if sign-in is successful. I love the Pangolin interface. The numbers and icons are so clearly displayed, it’s easy to dial a number. Quality is also very good. I’ve used it for like a dozen of calls throughout the day and the voice quality is comparable with my landline phone. The record feature comes in handy, too. You just press REC and your conversation will be recorded in WAV format in less than 1MB per minute. That means, it doesn’t consume too much space in your hard drive.
All in all, Pangolin™ gets my vote for simplicity and convenient features.