As users of the Problems of the Week, I thought you might be interested in taking a look at an experimental PoW project that is called ESCOT PoW (Educational Software Components of Tomorrow). We are in our second year and this year the problems are available using a regular browser.
Although it is necessary to download some software to get started, I think the problems have great potential! Each of them has a java applet page that allows students to interact with the problem. This is a middle school project but teachers have told us that upper elementary and also first year high school students might find the problems interesting and challenging.
In my role at the Math Forum I correspond with teachers who are using the problem and I have put some of these letters on the Web. They may have some interesting background information: