Working in a classroom with many different levels can be quite challenging. Finding resources that motivate all students is something that we strive for. One of the resources that I use is the Problem of the Week from the University of Waterloo. These have been great for my class. Each week they send out problems at different grade levels. I print out copies of all the levels and give the students the choice of which ones they want to try. The levels are grade based with Grade 5/6, 7/8, 9/10, and 11/12. I have some students who work at the 5/6 while others attempt the 11/12. The great thing for my problem solving class is that I don't have the answers for the problems and so I am able to work through some of the problems with the class. They get to see an authentic attempt at problem solving by me and see how I go about trying to figure it out with them. They also give their input when they see my thinking is wrong or question when something doesn't make sense. Waterloo gives you a week to figure it out before they put out their detailed answers. These have been great for the class as they get excited to see the answers and see if their thinking was correct and they are able to work at a level that is comfortable for them.
Here is the link: http://www.cemc.uwaterloo.ca/resources/potw.php
If you have a great problem solving website , please share.