The I amsterdam City Card provides free and discounted admissions to many of Amsterdam's top museums (but not the Anne Frank House), plus a free canal ride, free use of public transport, and a 25% discount on various attractions and restaurants—savings can be substantial. A 24-hour card costs €49, 48 hours costs €59, and 72 hours €69. It can be purchased at branches of the VVV (Dutch Tourism Board), the GVB (both opposite the Centraal Station), and at some hotels and museums. It also includes some attractions outside of Amsterdam, including the Zaanse Schans, Zuiderzeemuseum in Enkhuizen, Amsterdam Castle Muiderslot and attractions in Haarlem and Volendam.
If you are here for a few more days or intend to visit museums in other parts of the Netherlands, such as Rotterdam, a Museumkaart (museum card) might offer better value. This gives you free entry to 400 museums throughout the country for a year, including the top draws in Amsterdam. It is available, upon showing ID, at participating museums for €54.95.