Khinchin's constant to as many decimal places as possible in 64 bytes or less
CJam - 118

2'."*;TeT?_aN{.i9B*ZEay
G`9~eW}nE=Lr-`B}    )D&gt;9m9"136b


Try it at http://cjam.aditsu.net/

Since stackexchange destroys some of the characters, here's a program that generates the program above; run it first, then run its output:

"2'.\""685452001065306445309714835481795693820382293994462953051152345557218859537152002801141174931847697995153465905288090 136b:c"\"136b"


Explanation:

2 pushes 2
'. pushes the dot
"…" is a string containing the rest of the digits in encoded form
128b converts the string to a number, treating the characters as digits in base 128 (via their ASCII code)