Making Mathematics Word Problems

From site:

From 1999-2002, Making Mathematics matched students and teachers in grades seven through twelve with professional mathematicians who mentored their work on open-ended mathematics research projects. Although our project has ended, we encourage you to look through our mathematics projects, teacher handbook, and mentor handbook [PDF] for information on pursuing mathematics research projects with middle- and high school students.

There are 12 complex problems. They deal with number theory, algebra, calculus, geometry, patterns and reasoning. Each problem has warm-up problems and hints to help with the solution. There are also extension problems related to the original problem. So that actual number of problems available is much more than 12.