Teleo - internet calling for everyone, anywhere: "Teleo provides Internet telephony applications that bridge the gap between your computer desktop and regular telephones and cell phones. Teleo's software integrates with your computer desktop and allows you to place and receive telephone calls from Microsoft Outlook, Internet Explorer, and other applications. Teleo allows you to place PC-to-PC calls to other Teleo users anywhere in the world for free, even over wifi connections and behind firewalls. Calls from regular telephones are also free. Calls to regular telephones are paid as you go using flexible PSTN minutes with 2 [cents] per minute calling anywhere in the world. If you want to avoid using PSTN minutes, simply get your friends and business associates to install Teleo, and then you'll be able to place and receive calls from them for free, no matter where they are in the world."
(Thanks for the pointer, Irwin)