Remembering millions of digits of pi is trivial. Given time, I can cite a billion of them. Just don't ask me in which order they appear. Now for the useful constants I remember:
0 - first interesting number.
1 - last interesting number.
2 - number of eyes typical humans and penguins have.
3 - number of vertices of a triangle.
4 - number of players needed for a game of bridge.
5 - most symmetric multi-dotted face on a die.
6 - number of days in a week, not counting Wednesday.
6.5 - halfway 13.
7 - number of cardinal sins.
8 - only digit to divide the plane into three mutual exclusive subsets.