Here's a hidden treasure I hadn't heard about until now: The Ij Hallen Flea market. Just a short ferry ride from Amsterdam Centraal, this one-monthly flea market offers about 500 stalls of everything you can imagine. Here's a colorful description from CITIZN:

For a very small entry fee (3.50 EU for adults), access is granted to the largest trash and treasure market in North Holland. Stallholders come from across the low lands, and vary greatly in age and personality, from bored students, serious vintage resellers, aspiring entrepreneurs, cunning merchants, and lovely mother hen types that are clearing out sentimental possessions that are gathering attic dust.

2011 will see the market held on the following dates (open from 9am-4:30pm):

2 and 3 April
7 and 8 May
5 June (Outdoor)
3 July (Outdoor)
7 August (Outdoor)
4 September OR 10 and 11 September (dates TBC)
1 and 2 October
5 and 6 November

Check the website for more details.