Skype
I have been playing with Skype for the last few days.
It is a free voice over IP system: internet telephony. It works on a number of platforms: Linux (hooray!), Mac, Windows, and PocketPC. You will only have to pay if you want to call a non-Skype system, eg. a landline.
Sometimes a little choppy. Sometimes a little echoey. I would still recommend you take a look.
Add a comment
I've been part of this playing-with-Skype thing, and it can work quite well sometimes. Other times (say you try loading a website up) make it jump, go all metallic (presume it lowers bandwidth usage to try and keep the latency down) or jumpy. This is even on 512kb ADSL broadband which, you'd have thought, shouldn't need stretching to manage just a compressed (and encrypted) simple audio stream. When it works well it's pretty good quality sound - definitely better than a mobile, better than yer average BT landline. Just careful to not get any feedback/echo as said. Headphones+a good mic work best (suppose a headset would be better suited).