Some of my favorite Twain quotes have already been covered but I'll add:
A person who won't read has no advantage over one who can't read.
But who prays for Satan? Who, in eighteen centuries, has had the common humanity to pray for the one sinner that needed it most?
Clothes make the man. Naked people have little or no influence on society.
Don't let schooling interfere with your education.
Mark Twain was a great, great man. Now for some other quotes:
Family Guy-"You said nuclear, its nukular dummy the S is silent." Peter Griffin
"If it's a penny for your thoughts and you put in your two cents worth, then someone, somewhere is making a penny." Steven Wright
"So wait you just put the lobster, alive, into boiling water, and it screams and turns red and dies? That is the most metal thing I have ever heard." Pickles, the Drummer, Dethklok
"You can't blame television for your children's behavior. Haven't you tried taking your kids, sitting them down, and hitting them?" Bender
"An escalator can never break: it can only become stairs. You should never see an Escalator Temporarily Out Of Order sign, just Escalator Temporarily Stairs. Sorry for the convenience." Mitch Hedberg
"Atheism is a non-prophet organization." George Carlin
And to end on a more poignant note:
"The optimist thinks this is the best of all possible worlds. The pessimist fears it is true." J. Robert Oppenheimer