I would like to know if there is an online map or other reource that lists windmills in the Netherlands. Preferably ones that can be visited. If there information about their former function that would be a plus. I would like to visit a "water pomp" windmill.
There is a website called called allemolens.nl. "Alle molens" is dutch for all mills. Based on the name only, I would say that this is the resource that you are looking for. Unfortunately the page is Dutch only. It states
Hier vindt u informatie over alle molens in Nederland, bestaand en verdwenen.
Which translates to:
This resource contains information about all mills of the Netherlands, both the ones that still exists and those that got demolished.
For a non-dutch speaker the website might not be that informative, although google translate might help you a bit. They do have a search engine, but again, you need to locate a dutch speaker for it to be effective.
If you are specifically interested in windmills that are called "polder mills", that is that they serve a purpose in draining the gained land in the Netherlands, I can recommend the Zuiderzee museum. They have a mill in their collection and you can see the underlying principle by leveraging some water yourself.
The more touristic solution would be to visit the kinderdijk:
Source: Wikimedia commons.
There is an annual "windmill day" where alle the 950 windmills in the Netherlands will be in action. On that day you can visit many windmills. For 2014 the dates are may 10th/11th.