I never realized Albert Einstein's birthday was on Pi day, that's pretty neat.
I have heard some carpenters use the 22/7 approximation which is accurate to 2 decimal places and usually more than accurate enough for their needs. I found it amazing that Babylonians approximated Pi as 3 and the Egyptians using Pi approximation as 3.14. Of course at the time they diddn't know more.
A lesser known approximation with even better accuracy is the fraction 355/113 accurate to 6 decimal places. Although, remembering that or simply remembering 3.141592 seems to be just as easy. However 3.14159265 rolls off the tongue just as easy and is even easier to remember.
Apparently NASA uses Pi to 15 decimal places. At least that was a common answer they gave to questioning individuals. During number crunching using computers I find it hard to believe they would truncate Pi at 15 decimal places.
I started looking around to see if there was an algorithm that would find digits of Pi without having to carry all the previous digit calculations. Turns out an algorithm exists but only in hexadecimal, known as the BBP formula discovered in 1995 by David Bailey, Peter Borwein and Simon Plouffe (can't paste formula in at the moment). Unfortunately there's no analogy in decimal form, but it is argued that the amount of power required to determine those digits require just as much energy in calculating normally.