Problem Solving Resources
Mathematics teachers are always looking for new and interesting problems to share with their students (or to do themselves). If you aren't, then get with it!
A multitude of problem solving resources in mathematics are available on the Internet, in the form of problem collections, weekly problems, problem contests, etc. Rather than reviewing a specific site, I suggest that you visit two sites that focus on providing links to quality problem resources specific to mathematics.
For example, if you teach mathematics on the high school level, a good resource (but very bland visually) is the Park City Mathematics Institute (PCMI). It offers links to PCMI problems, Harvey Mudd problems, college math journals problem sets, mathematics competition sites, online problem resources, and textbased resources. Of the latter, I strongly recommend the very fun text Which Way Did the Bicycle Go? : And Other Intriguing Mathematical Mysteries by Joseph Konhauser and Dan Velleman.
Or, if you teach mathematics on the middle school level, a good resource (and better visually) is the MathForum. Though you may need to signup as a member for some of the problem resources, it offers links to algebra problems, prealgebra problems, geometry problems, brain teasers, etc. Just click on the link Browse the Internet Mathematics Library and you will have a world of problem solving resources opened to you.
