Virtual Filing Cabinets
These links contain organized lessons written by real teachers, a fantastic set of resources to aid in lesson planning.
Robert Kaplinsky put together a fantastic customized Google search that searches a number of great problem-based lesson websites.
Someone at the U of MN put together a great Google site of resources. The Search in the upper right corner only searches curated math lesson sites.
Sara Van Der Werf (MCTM president) put together a great list of resources; Her favorite math websites, her favorite math blogs, her favorite math videos, and her favorite (mostly math) people on twitter.
Infinite Sums: Blog with lesson ideas.