On: Google URL shortener available to the public

[c00p3r]

What hashing function they use?

[mahmud → c00p3r]

Base62.

http://stackoverflow.com/questions/1119722/base-62-conversio...

A case-insensitive variant is Base36:

http://en.wikipedia.org/wiki/Base_36

I remember you being a Lisp programmer; it's built into CL's Format function:

  (format t "~36r" 9999) ==> 7PR