Low cost prepaid calling in Germany and France?

[inquisitor]

For Germany I'd recommend Fonic, since their national and international rates are very cheap (€ 0.09/min to Germany and the US) and their SIM is quite widely available. For details check Germany - Fonic - Prepaid Wireless Internet Access

Roaming with a German SIM in France doesn't really make sense. You'd have to pay € 0.17/min for inbound calls and outbound calls would be more expensive than with a French SIM. In this posting I listed a couple of French MVNOs with cheap national and international rates. It seems like Mobiho (formerly "Primus Mobile") is the only one selling their SIMs offline (retailer search). Interestingly Mobiho has cheaper international rates than national ones.