Ajax Stadium
Take a guided tour of the famous soccer stadium.
http://www.amsterdamarena.nl/

ArchEX
Architectural tours of Amsterdam.
http://www.archex.info/

Dorling Kindersley
Download a free audio tour guide to the city, courtesy of DK.
http://traveldk.com/about/podcasts

Double Dutch Events
Arranges various tours and activities for groups ranging from 8 to 500 people.
http://www.doubledutchtours.com/

Lindbergh
Offers tickets, excursions, tours, even accommodation and transfers for group and individual travellers alike.
http://www.lindbergh.nl/

Private View
Personal tours highlighting Amsterdam's culture and history.
http://www.privateviewtours.nl/

Simply Amsterdam
Index of various tours and excursions on offer to visitors to the city.
http://www.simplyamsterdam.nl/services.htm