Popular chat app imo has added free voice calling to the most recent incarnation of its app for iPhone, iPad and Android. imo users can send and receive voice calls without using cell minutes, though both the caller and callee have to have the app on their device.

The imo app began life as an IM/ chat aggregator, allowing you to centralize all your contacts and chat with everyone in one spot on your desktop and/or mobile device. Group chat is still supported in the new version, as is Skype, AIM, GTalk, MSN Messenger, Jabber, Facebook chat, Yahoo chat and more. imo also offers the “voice IM,” which allows users to send 90 second voice messages instead of actually typing – kinda like an updated version of the Nextel phone/walkie-talkie.

imo isn’t the only group chat or chat aggregator out there, but it does offer a social twist: imo wants to encourage people to make friends through the service too. But don’t worry, it’s nothing like Chatroulette. It’s more like a very temporary bulletin board, where users can ask and answer questions just to see who’s out there.

AllThingsD has a great interview with the founders, brothers Ralph and Georges Harik, about their vision for the service. Otherwise, imo is available for iPhone, iPad, Android, Blackberry, Nokia and Windows Messenger.

Have another app for free voice calling that you like? Let us know in the comments!