Some of my favorite links:

• Types of Waves - Really cool animations of waves
• Science Fair '07 Photos
• "The Elements" song
• Another Periodic Table of the Elements
• Dictionary of Chemical Formulas - Wikipedia.org
• Vladstudio - Best wallpaper on the internet
• Moustrap-Powered Vehicle Challenge Results: 2008 | 2007 | 2006
• TCS Student Zone
• Bill Nye the Science Guy - science rules!
• MarshmallowPeeps.com - who doesn't love Peeps?
• Science Museum (Florence, Italy) - see Galileo's finger!
• Snowflake Science 
• Chem4kids.com - The Elements  
• Chem4kids.com - Atoms
• Balancing Chemical Equations - A good site for practice.
• Quarks, gluons, and more - what are atoms made of?
• The Wooden Periodic Table - A great visual periodic table.
• Mendeleev's periodic table - Where it all started.
• Position-Time Graphs 
• Your Weight On Other Worlds 
• Docphizzix.com - A resource for mousetrap-powered vehicles.