O2 has launched a voice and messaging app under the odd name of Tu Me, which offers users of all mobile networks an alternate, free way to make and receive voice calls.
You will need a data connection or be in range of a usable Wi-Fi hotspot to use it, meaning all calls and messages sent from within the Tu Me app will use your mobile data allowance or home internet connection, rather than your contracted minutes and texts. Plus whoever you're planning to bother will need the app installed as well, as there's no option to call regular landline numbers as there is within the popular Skype.
And the other pretty big caveat at the moment is the way Tu Me is currently only for iPhone users, although the Tu Me site has a little notification area on it, hinting that it may appear for other devices at some point in the future. [Tu Me]