Open-ended problems

For one perspective, see the book The Open-Ended Approach: A New Proposal for Teaching Mathematics, edited by Jerry P. Becker and Shigeru Shimada (National Council of Teachers of Mathematics, 1997).

Links (not all interpreting "open-ended" the way Becker and Shimada do):

Franklin Institute

Akihiko Takahashi web site

University of Michigan

A searchable collection of open-ended questions