Clangpics - make pictures from C expressions.
Pinboards - stick pins in pictures and maps.
See how your constituency moved between 2001 and 2005 in the three-way contest between Labour, Conservative and Lib Dem.
Online matrix division lets you divide and invert matrices and solve linear systems. For when you're on the road without your favorite Matlab clone and you just have to do some linear algebra. (This actually happens to me quite a lot!)
Web-based social networking from the early 90s: Whimsy.