Emacs (available at http://www.gnu.org/software/emacs/) is the granddaddy of text editors and is probably one of the oldest of all. The beauty of Emacs, however, is that it is one of the most customizable editors out there, so much so that not only can you write code in almost all of the programming languages you can think of but also carry out other activities such as using it as a Twitter client, e-mail client, or a directory editor. Emacs has a major mode which tends to be language or task specific (such as Clojure, Ruby, Haskell, Python, and many more), but only one major mode can be active at any given time. A minor mode is an optional editing mode that alters the behavior of Emacs in some well-defined way.

Many minor modes can be active at any time, so for example, along with some major mode, the auto-indent minor mode can be active, or we can use the Paredit minor mode (which keeps the parentheses balanced) while working with S-expression based programming languages (such as...