I'm moved to the US, and found Skype2Go/Skype out to be one of the least expensive options for calling from my US mobile to a UK mobile, yes, you can get deals on international calling from US mobile to UK landline, but interantional mobile to mobile, well I couldn't find many lower cost options, and I refuse to give AT&T any more many than I need.
I used Tango now as well, if I'm on WiFi, and the other person I'm calling is on WiFi, it is in esssence free, and has few neat tools, I also found the audio better than Skype.