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.

Thankfully, there are many similarities between Skype and instant messaging (IM) applications, with which many people are already familiar. For instance, you can add your friends, hide your online status, block callers, track calls, have an IM-style chat or take part in a conference call with up to four others.

On the voice front, Skype works by converting the sound of your voice into a digital data packet, which is then fired across the internet and converted back into sound at the other end.

Skype will work across a modem (33Kbits/sec or higher), and all you need is a microphone and some PC speakers or headphones.