I wrote data only, because they don't (yet) support roaming and calling abroad. Calling within The Netherlands of course works and costs €0,20/min.
This new provider charges
€0,50/day for unlimited internet which they claim
really unlimited - I couldn't find anything about a FUP or something like that in their TOC. The connection is throttled to 386kpbs on the T-Mobile network - the worst of the three networks in The Netherlands, but it's fine for usage in cities.
They also have a €1,00 per day variant which loosens the throttling up to 3,6 Mbps with a maximum of 2GB (after that they throttle again to 386kpbs).
You only pay for the days you use - you can choose online if you want to use the €0,50, €1,00 or nothing at all.
Validity: you need to make one sms/call every 6 months
The sim cards can be bought at all Free Record Shops in The Netherlands for €10 (including €10 credit on the card). I bought one myself today to test it out and it all looks very nice