Here are some excellent resources across the curriculum. These are website to put on your favorites, assign quests, centers, etc. Add to this as you find additional resources!

Math Playground
Math Dictionary for Kids
Virtual Manipulatives
Stowe-Munroe Falls Investigations
Investigations Checklists
Stuart J. Murphy
Math Teacher's Toolkit

edudemic's list of the top 50 educational tools. Sept. 2012

The Super Book of Web Tools for Educators