Nowadays, most accommodations, bars, clubs, restaurants and museums in Amsterdam have free WiFi for their guests. The name of the network and the password are either written on the wall somewhere, or you just ask for it.

Special mention deserves:

There is free WiFi in intercity trains in the Netherlands. The network in the train is called 'WiFi in de trein'. You need to log in, but no password is required, just accepting the general conditions is necessary. To accept this, you select a tick box.
The login page is in Dutch, so there's a bit of guess work to do. If you get lost, try to ask help from a local - you're rarely alone in the train.

The service is only in intercity trains (the Wifi logo is on the outside of the train), not in so-called sprinter trains - trains that stop at all train stations.

On Schiphol Airport there is free internet for one hour. For more time there is a paid service.

There is free WiFi in the line waiting for the Anne Frank House.

Also, some parks have free WiFi. The larger part of the Westerpark has free internet, and there is also free internet in the Oosterpark, but only around the children's pond at the North end of the park.

Most Albert Heijn and Jumbo supermarkets have free WiFi.