My fiancée and I use Rebtel. It's a free app available for Android, (probably iPhone and others too). As long as you both have it installed, you can dial your international number and you won't be charged. It will however use up your bundled minutes. Also, it uses both parties' minutes, regardless of who initiated the call. However, as long as you have minutes available as part of your tariff then it's a great way to stay in touch
Incidentally, for anyone calling the US from the UK, there's an 0844 1p/min number you can use: 0844 720 2443... you call that number, then at the prompt dial the full international number (including 00) - you only pay for the 0844 call! I use it to call my fiancée when I've used up all my bundled minutes
It's 1p/min from BT landlines, but if you call from a mobile make sure you check how much you'll be charged for an 0844 number first.