Vodafone and 3 offer free calls to other users on the same network for many contract cap customers. The Who2Call application lets you know who in your phone contacts uses Vodafone or 3, meaning you can call them without charge if you're on a suitable plan.
It's a simple idea, but a potentially useful one if you do have a large contact list and can't remember who might qualify for charge-free calls. Who2Call is a free download for iPhone and Android phones. Note that while the calls may be free, the data used to update the application (which happens by default once a week) will come out of your data allowance.