A-mobile (Auchan Mobile) started operating on October 16th on the SFR net.
Their offer (only in french).
The calls to France, Europe and North Africa are 0,40? per minute.
SMS are 0,10? each and MMS are 0,45? each.
Carrefour is going to start is offer on November 8th.
They announced that calls will cost 0,39? per minute.
At the moment I have no more information...

Unfortunately the credit runs out quite quickly (the 35? card lasts 90 days, which is the longest) like all the other French prepaid cards. Still, it seems like a better offer than most for tourists visiting France for a short time, although Auchan tend not to be in touristy areas. If only there were more real competition, though, rather than the three networks being able to keep MVNOs on a very short leash...

I think the Virgin offer is better: the credit expires after 3 months even if you top up 10? only.

This is the Carrefour mobile site.
Calls to France are 39 cents per minute.
10 cents per SMS (20 cents if sent to a foreign country), 30 cents per MMS.
Nothing really new...

I saw a spot on TV about M6 Mobile.
They launced the prepaid card, sold in the newspapers shops but I think that is an Orange rebranding.
Anyway this is their site.

It doesn't look like tariffs are heading downwards at all, so people would probably use a callback service with any of these. Though Auchan has calls to abroad at the same tariff which would be ok for calling foreign mobiles.

The Carrefour one looks like it would be cheap to keep alive, ?4 minimum top-up possible online (foreign credit cards?), and able to receive calls for 6 months.

Did I see somewhere that the NRJ card now expires in a certain time without credit, rather than staying alive by making or receiving calls like UK ones?

Yes, NRJ has changed the credit validity to the standards of the market.
No way of keeping the card alive by making a call every 6 months!