Skype is software for calling other people on their computers or phones. Download Skype and start calling for free all over the world. The calls have excellent sound quality and are highly secure with end-to-end encryption. You don't even need to configure your firewall or router or any other networking gear.
Additionally it doesn't just work on Windows. Skype is also for Mac OS X, Linux and PDAs using Pocket PC, with a native look and feel for each platform. Talking, sending instant messages or even file transfers work between different platforms like a charm.