T-Mobile probably has the best options when you’re looking to hook up your laptop to the mobile web in the Netherlands.

The standard prepaid plan: unlimited access at € 4.50 per day
This plan is enabled on the "garden-variety" prepaid SIM cards. You can get one for € 10 with € 5 calling credit, plus € 10 after on-line registration.
With these SIM cards, the cost of using mobile internet is deducted from the prepaid calling credit. The costs of T-Mobile PrePaid web’n’walk are € 0.20 for each unit of 200 KB. You will never have to pay more than € 4.50 per day, however. This gives you unlimited access at UMTS speeds.

The "Laptop Prepaid" plan: even faster unlimited access at € 7.00 per day
T-Mobile also sells special prepaid SIM cards under the "Laptop Prepaid" plan. These can access the web at greater speeds (2048/384 kbps), but have a higher maximum per-day charge: € 7.00. Otherwise, this plan does not differ much from the above plan. The cost of using mobile internet is deducted from the prepaid calling credit. The costs of T-Mobile internet are € 0.20 for each unit of 200 KB.

For details see the pricelist here.

Account Info (taken from here and a minor update from t-mobile.nl here):
APN: internet
Username (not sure if it is necessary): tmobile
Password (not sure if it is necessary): tmobile

Roaming: Cost per MB with a Prepaid SIM when roaming outside the Netherlands: € 1.68 per MB