Website: HowToSmile.org

I recently learned of a new (brand new to me, fairly new in general, I think) website filled with science (and math) activities: HowToSmile.org.

Currently, most of the activities on the site come from the collections of various hands-on science museums (The Exploratorium, Science Museum of Minnesota, Lawrence Hall of Science, etc.). 

I particularly like that you can create your own lists of activities - create one list of everything that looks interesting or create several lists and sort activities by topic, grade level, priority, etc.  Possibly even better than that, is the ability to search other people's lists.  I tend to find the most exciting ideas by looking at what other people have deemed worthy to add to their lists.