We've featured messaging/VOIP app Viber in its iOS and Android incarnations before. Version 2.2 of Viber enhances the app on those platforms with the ability to make group calls and send photos in messages.
Like other "free messaging" apps, Viber relies on your friends and family also using it (it claims a user base of 90 million people so that may not be a massive problem). Remember also it's not 'free' in the strict sense, since you'll be using part of your data allowance, or relying on Wi-Fi that someone else is ultimately paying for.
Viber 2.2 is a free download for Android and iOS; BlackBerry and WP7 versions are also available, but are further back in the development cycle. If you've had a good (or bad) experience with Viber, tell us about it in the comments.