There are literally thousands out there, but these are some that WUSD teachers have shared over time.
General Resources:
National Library of Virtual Manipulatives
Federal Resources for Educational Excellence
Common Errors in English
180 Technology Tips
How Stuff Works
Annenberg Media
Smithsonian Institution
LiveScience
Open Library
Library of Congress
Mathway (step by step solutions to problems)
Create a Graph
Technology in Education Resource Center
Gapminder (statistical representations)
Footnote (historical documents)
Teaching That Makes Sense (great resources for writing)
Language Translation Resources:
Babelfish
Music-Related Resources:Musical Acoustics Training:
UNSW Acoustics
Online Simulations, Games and Labs:
BioKids
Cells alive!
Chemistry Online (University of Oxford)
The Chem Collective
The GLOBE Program
Interactive Science Simulations (University of Colorado)
Council for Economic Education (Financial Literacy and Econ)
Virtual Field Trips
The Virtual Smithsonian
Jet Propulsion Lab at NASA
National Geographic Expeditions
Ball State University
Colonial Williamsburg (best for grades 4-8)