Paredit is an Emacs extension for working with parenthesis and S-expressions.

From the ParEdit page:

ParEdit (paredit.el) is a minor mode for performing structured editing of S-expression data. The typical example of this would be Lisp or Scheme source code.

ParEdit helps keep parentheses balanced and adds many keys for moving S-expressions and moving around in S-expressions.

Seeing it in action makes it much clearer how useful it is. Here’s two great visual guides to ParEdit and what it can do:

  1. Emacs Rocks! Paredit
  2. The Animated Guide to Paredit

Like most things Emacs, it takes a while to get used to. But once over the initial learning curve it’s indespensible.