Will I sound like a total GC dork if my favorite quote is one that I learned from my Fraternity's probationary member manual? I have used it as a sig from time to time; it is attributed to Plato:
The good man is the only excellent musician, because he gives forth a perfect harmony not with a lyre or other instrument but with the whole of his life.
Shakespeare provides so many quotable lines, but like SWTXBelle, I seem to be using one in particular from Hamlet a lot:
There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.