Eurolines is the main ‘operator’ for international coaches to the Netherlands. (In fact the name Eurolines is a common brand-name used by different operators). Services are limited: only a few main routes have a daily service, such as from Poland, London, Milan, Brussels and Paris, but this is the cheapest way to travel and you get a discount if you are under 26 of age.
Megabus runs lines from London and Paris via Brussels to Amsterdam.
PublicExpress runs a line from Bremen via Oldenburg to Groningen.