Boy, has the meaning of this word changed!

Nowadays, "curmudgeon" is used exclusively to mean an opinionated old fart who is angry that young people don't value the things he/she does. A curmudgeon usually has a grim, pessimistic vision of the future and his fellow man.

Famous historical curmudgeons:

Jonathan Swift
Richard Francis Burton
H. L. Mencken

The most famous curmudgeon today is probably Andy Rooney.