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:
Math Playground
Math Dictionary for Kids
Illuminations
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