Just got back from a family vacation, and while in France we picked up a couple of prepaid SIMs at a tabac from a carrier called Mobiho,
http://www.mobiho.fr/
15 Euro with an 8 Euro credit; 0.29 per minute to other Mobiho customers; 0.39 per minute to land lines (and 0.18 per minute to USA). Easy to set up and good signal everywhere we went -- no idea who the actual network provider might be.
--Steve