Emacs Flymake

Emacs Flymake is an excellent plugin for Emacs that adds on-the-fly syntax checking for a number of languages.

Originally by Pavel Kobyakov, my fork on GitHub is a replacement for that distributed with Emacs, adding a number of bug fixes including support for limiting the number of parallel invocations of syntax checks and spawning a single timer so that it plays nice with desktop mode, and support for using a temporary directory for the syntax checks rather than the same directory as your source code.

There's also tighter integration with my fork of Emacs Flymake Cursor and my Emacs Flymake PHPCS and Emacs Flymake Perl::Critic projects.

You can find more on my GitHub page for Emacs Flymake.