Flycheck
On the fly syntax checking for GNU Emacs
...It uses various syntax checking and linting tools to automatically check the contents of buffers while you type, and reports warnings and errors directly in the buffer, or in an optional error list. Out of the box Flycheck supports over 40 different programming languages 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. ...