This is an excellent Peer2Peer VoIP system, I've even been able to get skype working in linux.
Skype has great features like SkypeOut (Make outgoing phone calls to landline phones for a cost), SkypeIn BETA (Have a landline phone number), Skype VoiceMail (Answer service that comes free with SkypeIn BETA) and finally Skype Zones BETA for an extra price it allows you to use Skype when wirelessly connected to a Boingo compatible Wireless Access Point via a laptop or pda. Skype also uses encryption for secure phone calls, text chat and file transfer.
Here are various links from the Skype site...
Skype for Mac OS X/ http://www.skype.com/go/getskype-macosx.dmg
Skype for Linux (You have to pick manually)/ http://www.skype.com/products/skype/linux/
Skype for Windows/ http://www.skype.com/go/getskype
Skype for Pocket PC/ http://www.skype.com/go/getskype-pocketpc