A table of 120 known constants in math, such as Pi, e, sqrt(2), parking constant, Feigenbaum constant, etc. each of them is with references, and up to 1024 digits when possible can be found at the Centre for Experimental and Constructive Mathematics, Simon Fraser University at http://www.cecm.sfu.ca/projects/ISC.html
Another source of mathematical constants is:
http://www.mathsoft.com/asolve/constant/constant.html
maintained by Steve Finch.