Skype is a proprietary software application that offers free Skype-to-Skype calls and low-cost local, national and international calls to both traditional land-line telephones and mobile phones. Additional features include instant messaging, texting, call forwarding, and video conferencing.
Skype also has a premium version with support to Group Video Calling.
On Skype for iPhone, you can make free video calls to other devices with Skype installed, over both Wi-Fi and 3G.
For webOS phones: https://developer.palm.com/appredirect/?packageid=com.palm.app.skype
Available also as portable app: http://portableapps.com/apps/internet/skype_portable
Mac, iPhone, iPad, Windows, Windows Phone, Windows RT, Android + Tablet, Linux, Online
+4 more Blackberry, Windows Mobile, Symbian S60, HP webOS