National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: About Pi
This site provides an in-depth discussion on what pi is. It gives a brief history revealing what the Egyptians and Babylonians estimated pi at.
National Council of Teachers of Mathematics
The Math Forum: Pi Day
Need some ideas for Pi Day on March 14th? You can get some great resources at this location. There are all kinds of links available.
University of St. Andrews (UK)
University of St. Andrews: A History of Pi
From a historical view, the authors give a very descriptive timeline of pi.