Skype
I downloaded Skype this evening and had a play — it’s amazing! It Just Works™! It’s an internet telephony application: you plug in headphones and a microphone (or a headset) and you can talk to other skype users over the internet in real time, for free. I was very, very impressed with how easy it was to set up: a single download, type in your name, and voilá, you’re online and ready to make calls. No registration, no complicated setup, very easy.
The really neat feature is that they have an interconnect to the old phone system too, so you can call real telephones. They're a pretty reasonably-priced telco, too: it costs two euro-cents (about 1.5p) per minute to call a New Zealand landline! They also do traditional text-based instant messaging, like Jabber or ICQ. I'm looking forward to using the €10 of call time I bought, but it'll have to be on the Windows machine at work since there's no Macintosh client — yet.