Num Sum: Here’s a great little app for creating small, simple speadsheets online and sharing them.
The Pizza Calculator shows you the price per square inch of pizza based on the size you buy — it’s fully a third less per inch when you buy a 20-inch over an 8-inch, assuming his pricing model is correct.
The Temperature Convertor is handy.
Here’s a simple matrix of Julian dates for 2005.
Besides the gee-whiz factor, here’s how this is valuable: it can teach theory. With something like this, you get an easy way for someone to explain how a particular formula or theory works. Instead of just reading about it or just looking at a flat report of it, they can experiment and being to understand.