3/14 is considered to be Pi Day by many geeks, as π is approximately 3.14. I wish I could say that I had celebrated by eating pie, but I didn’t.
π is surely the most important irrational number, which has been known since antiquity.
π shows up in so many different formulas. My own favorite is Euler’s Identity, eiπ + 1 = 0. (This marks the first use of the reStructuredText math role in my blog.)
I am also find of the approximate fraction 355 ⁄ 113.
The Wikipedia article on π has much more on π.
Update: Wonkblog: 10 stunning images show the beauty hidden in pi