At a Math Resource meeting today, I learned about a site that is absolutely perfect for the Problem Solving element of Math Workshop. I know that one of the most challenging aspects of trying any new instructional strategy is gathering resources, so this site should really help with that.
The site is part of insidemathematics.org and can be accessed by clicking here. The site is free, and is leveled by grade and standard. Not only is there a kid-friendly problem for every topic, but they are standards-based and include explanations of the problems and solutions. There are also options for leveling.
Hopefully this will be another great resource to bookmark for your Problem Solving needs.