One of the "must sees" if you visit The Netherlands in the spring is the Keukenhof. Somewhere in the neighborhood of 7 million bulbs are planted annually so it's a wonderful treat to see them all coming up. According to their website, there are 100 varieties of tulips there.
When you visit, make sure to wear comfy shoes as there are 15 km (9.3 miles) of walking paths. I understand it is also possible to rent a bike and cycle through the flower fields surrounding the Keukenhof but I haven't had the opportunity to do that yet.
This is a mosaic that I made from some of the photos I took last year. To see the rest, you can check out my flickr set here.
Amusingly enough, tulips aren't even actually Dutch, but were imported from Turkey in the 1600s!