Jetbrains PhpStorm

PhpStorm is a (commercial) editor with many of features and used by most Joomla developers. It's free for students and for open source projects there are also free licenses available. Niels Braczek is managing those for Joomla.

It's available for all major operating systems (Linux, Mac, Windows)

Installation

Just download and install from JetBrains website (There is also a 30 day trial).

Alternatives

Netbeans, Eclipse, Sublime, vim or any other editor with code highlighting.