I just read a posting concerning phone rates to the US. I've been using a company called 3U (www.3U.fr) for over a year without any problems. Their rates are extremely competitive, the access and quality generally good. The only drawback is that you must dial the prefix 1645 before any call.
Rates to the US are 5.9 eurocents the minute, and it is billed by the second, beginning with the first second.