Science has started a new series looking at blogs. The winner of the Science Prize for Online Resources in Education is http://learn.genetics.utah.edu/. On the right most panel is a Most Popular heading. Go to the Cell Size & scale and open it. You have to drag the tab on the bar, then you can scroll down to smaller and smaller stuff. Cool!
One of the scientific tools I use a lot in my job is electrophoresis. Under Virtual Labs on the left is a gel electrophoresis box. Run the demo. It's fun. I actually use this technique in an automated instrument with a disposable chip using microfluidics, but the demo is fun. I could waste a lot of time playing with this site.