"A Creative Encounter of the Numerical Kind" -- students develop a base-4 number system. Not very open-ended. Middle school
The Dilemma of the Dangerous Meat Loaf: Students investigate healthiness of meat loaf (or could be any food) and debate. Requires defining "healthiness", finding nutritional information, doing conversions, etc. Could be expanded to incorporate environmental footprint
Links to webquests in many content areas for different grade levels. Includes numerical evaluations.
Bernie Dodge, who is generally creditied with coming up with the idea of webquests, maintains this website. Includes background information on what webquests are all about, info on creating webquests, and a library of webquests (although I'm not sure it that works -- I searched for "math" and it returned no results.
Lots of links to webquests, organized by subject. A lot of the links I looked at are broken though.
A library of webquests. The couple that I looked at are very good.
This looks like a really simple-to-use and powerful webquest creation tool. And it's free.