From the USA

Continental, Delta, United, US and Martinair operate many flights to AmsterdamKLM and NWA operate many flights to Amsterdam
There are approximately 80 daily non-stop flights from all over the USA to Amsterdam Schiphol Airport. Airlines operating those flights are Continental Airlines, Delta Airlines, United Airlines, US Airways, Martinair, KLM and Northwestern Airlines. So (frequent) flyers can choose whichever airline suits them best.