Flycheck
On the fly syntax checking for GNU Emacs
... with more than 80 different syntax-checking tools, and comes with a simple interface to define new syntax checkers. Many 3rd party extensions provide new syntax checkers and other features like alternative error displays or mode line indicators. Flycheck needs GNU Emacs 24.3+, and works best on Unix systems. Windows users, please be aware that Flycheck does not support Windows officially, although it should mostly work fine on Windows.