This isn’t, technically, an educational site.  But I definitely think that it could be integrated into math classes everywhere, and even high school physics.  Remember the movie, Up?  Well, Movato Real Estate decided to find out how many balloons it would really take to lift a house.  On this site, David Cross explains the logic Movato used for making this estimation.  If I were using this with a class, I would not show them the explanation (especially as the Playboy Mansion is mentioned as an example), but would ask them to derive their own method for performing this thought experiment.  Remind them of the different variables involved including the weight of the house and the size of the balloons.  Depending on the age of the students, you could give them some of the information included in Cross’ explanation, or give them links to the references that are cited in the article so they can research on their own.  Finally, you can have them give an educated guess about the number of balloons it would take for their own house to be lifted (possibly using Zillow.com to research square footage, if necessary), their school, or one of the famous buildings that can be found in the interactive.  Have them put in their numbers, click on “Up, Up, and Away”, and presto!

Movato gives you an embed code for the interactive, so you could easily post it on your own website for the students to use.

Coincidentally, when I was about to publish this post, I found this article describing a real-life re-enactment of Up, and an even more ambitious goal for the future!