When it comes to making free calls across the internet using Voice over IP technology, Skype is certainly leading the pack. With almost 26 million people having downloaded the software, Skype is the most used free internet application.
Developed by the people behind the file-sharing service Kazaa, Skype offers free internet telephony with another PC user as long as both have the Skype software.
Of course, you're not limited to making calls, either. If you can see another Skype user online, you can request a text-based chat, in the same manner as you would through Windows Live Messenger or similar applications. If they're not at their desk, they will see your message as soon as they return.
This is the Mac version of Skype.