Great for homework. Michigan's Instrument Encyclopedia features world instrum. Here's an opportunity to run or derail a roller coaster. In addition, view the sky guide and send an e-card. Franklin Institute Science Museum HHMI's BioInteractive Lab - Very cool.
Site has interactive learning modules with animated clips.
Check out the 3D heart. How Stuff Works - Includes a search option. Provides links to related websites. Try their online calculator and grapher. Great for homework.