Moving to France, which cell phone plan and SIM card?

Google have/had a service that gives you free calls to most of the US and Canada (Landlines) and WA or other apps sort out mobile phones.
A Skype account can give you a local US landline number that will allow incoming calls to be diverted to your phone, and they have call plans that will allow you to call the US very cheaply.

We have had experience of 3 different SIM cards. Red by SFR - which is a cheap contract at 12 Euros per month and seems pretty good.  Setting it up was not terribly user friendly - it needs a (preferably) French bank account to direct debit monthly, but it is good value. Really cheap ones are Lycamobile - it has a budget plan of unlimited calls and SMS in France plus 2Gb internet for 4.99 Euros per month - even does roaming within Europe, and you can pay with a foreign credit card, or Lebara which seems a bit better organised than Lycamobile - and will direct debit foreign accounts - we have one for 10 euros per month and it works well and automatically tops up. You can subscribe online and the SIM gets delivered by mail pretty quickly (to a French address)