The Netherlands is, or at least was when I last heard about it, the number one producer of XTC (extacy), as well as the country from which most of those pills in the USA come from. They're said to be "better" than those made in other countries, meaning nothing more than that they have a stronger effect but cause as much, if not more damage.
There used to be a special bus at those large parties for some time, where people who bought XTC could have the pills tested. One thing I remember they said on TV was that they found several pills with rat poison in them. Imagine taking one or two of those pills...
Oh, and XTC in combination with alcohol can be deadly. Several people have fainted, had strokes, and some have been in coma from that.
The effect of XTC is that it gives you a boost of energy, making is easier to keep dancing all night long. The side effect is that your body temperature rises so you burn a lot of energy and tend to dry out, meaning you need to drink a lot (that's where the alcohol comes in). That's why you often see people who attend those kinds of parties with candy, such as lollipops.