Just out of nerdy curiosity, I dropped a couple of Google searches:
hotspot amsterdam
and
wireless internet amsterdam
Both came up with a number if sites worth checking out - some commercial, some free - and one even has a navigatable hotspot map. Most of the most useful sites are written in NL, but are still pretty easy to navigate.
Try these:
http://www.amsterdamwireless.net
http://www.amsterdam-wireless.nl/oost/moin.cgi/FolDerEn
http://www.hotspotamsterdam.com/
There was even a hotspot list on one of the sites:
Bagels and Beans - Waterlooplein 2
Albus Grand Hotel - in Deluxe Rooms - Vijzelstraart 49 - 31-0-20-530-6200
The American Book Center - Kalverstraat 185, 1012 XC - +31(0)20 625 55 37
THE FLYING PIG Downtown Hostel - Nieuwendijk 100 - 1012 MR - 31 (0) 20 4206822
THE FLYING PIG Palace Hostel - Vossiusstraat 46 - 1071 AJ - 31 (20) 4004187
NEMO - Oosterdok 2 (naast Amsterdam Centraal Station) - 0900 - 91 91 100
De Balie - Kl. gartmanplantsoen
Hope this helps!