I've made a new post today that brings the information in this one up to date. I've also included an FAQ section there that distills the content from the comment thread on this post. Instead of reading this post, which is now slightly out of date, and then trawling through 80 comments, may I suggest you go directly to the new post where you'll find all that information more easily. (Comments on this thread are now closed).
Please visit: Télépéage - version 2
The French motorways are usually toll roads. The amounts you have to pay are not a round number; Calais to Paris or Rheims costs €20.20 at the moment. So you're either trying to find the right change, on the wrong side of the car, or you're using a credit card for each transaction. Depending on your route through France, you may not just be paying the once; sometimes you have to pay for the stage you've just driven, then pay again a bit later on. Our recent French holiday to the south of Burgundy involved paying 3 times each way - 6 transactions in total. The other inconvenience is the queues. A lot of traffic travels on the French motorway network, and that creates a bottle-neck at the gares de péage, the pay stations.
There is an answer to this, and it's called télépéage. Similar to the gadgets you can get for the Severn Bridge, the Dartford Crossing or the M6 Toll, this is a small electronic transponder that you attach to your windscreen just behind the rear-view mirror. As you approach the barriers, a device by the barrier can read your gadget, extract the account details from it. You are let through, without having to stop, and you get billed later. Surprisingly, even lots of French cars don't seem to use them, but they speed up and simplify the journey considerably. This no longer needs to be a little secret kept by the French.
There are two ways to do it. The cheapest way is to deal directly with the French. The company APRR run the motorways to the south and east of Paris. They will let you have a toll reader from them, and bill it to an English credit card; most other French motorway companies won't do that. Whenever you get a télépéage gadget from one French motorway company, it works throughout France, so it's just a matter of finding a company that will let the English have one. APRR is your friend at that point. They charge you a (refundable if you return it) €10 for the gadget; they then charge you €1.50 per month, but only for those months when you use it. Other than that, you pay the same tolls as the cash price. There's no charge when you don't use it. You sign up at http://www.aprr.fr/fr/souscrire_telepeage_liber_t. But you need a little French to do it!
The not quite so cheap way is to use an English company, http://www.tolltickets.com/. They charge you €7.90 to send you the device, and you don't get that money back - it's a fee rather than a deposit. They also add 9% on top of the toll charges. So it costs more, but they're English. It's up to you.