Some really interesting and fun ones for kids (little and not so little)
Cells Alive (Need to learn or review your cell biology – great link to do that here).
Interactive Body (requires flash player)
Playing with Time (Time lapse videos)
Interactive Periodic Table – Really neat – besides giving you info on the site, when you click on each element and it takes you to a Wikipedia entry for that element
Understanding Genetics from Stanford University (including fantastic online exhibits: Zooming into DNA from 100X to 900,000X and What Color Eyes Will Your Children Have).
The Micronaturalist’s Notebook (wonderful photography of life forms under the microscope!!!)
PestWorld for Kids has some great games and identification guides for amateur entomologists.
Free Science Unit Studies/Curriculums
The Energy Story – A wonderful and free 20 chapter resource for learning about energy, complete with pictures and links for more information about the topics covered.