Many VoIP providers offer free softphone to make call from PC-to-PC or PC-to-Landline or Mobile.

ITelFone Softphone allows you to use your computer as phone. It is free to download and easy to install. With ITelFone Softphone you can take advantage of all calling features that traditional phone supports.

http://www.itelfone.com/softphone.php

Google talk

You can also use google talk gadget online to make calls, no download is required.

http://www.google.com/talk/index.html

With Skype’s free software – by the way, it works seamlessly with your internet connection – you can chat away with free Skype-to-Skype calls and never worry about cost, time or distance. It’s free, easy and quick to get on Skype.

http://www.skype.com/intl/en/download/skype/windows/downloading/

Globe7 Phone enables you to make Free PC-PC Calls and cheapest calling to any landline or mobile across the globe. You can also get the benefit of Instant Messaging!

http://www.globe7.com/downloadg7.php

PC2Call Calling Cards offer high quality voice services at the most competitive rates to all our callers. All you need to do is, simply download PC2Call dialler and make cost effective calls to anywhere in the world through your PC via Internet.

http://www.pc2call.com/downloads.htm

Yahoo voice offers following 3 services to make and receive voip calls.

You can make call anywhere in the world from your pc to landland or mobile.

Let friends call you on your PC - using their land line or mobile phones. You can pick your area code, so friends still enjoy local rates when calling you wherever you are

Call your friends for free with PC-to-PC Calling in Yahoo! Messenger. Just download Yahoo! Messenger,

http://messenger.yahoo.com/download