paredit

Taylor Campbell's paredit is one of those Emacs extensions that I can't live without. In a nutshell, it forces you to deal with Lisp code exclusively via operations that don't introduce unbalanced parenthesis (or other similar violations of structure). The genius about this approach is that it completely eliminates the step of making sure parentheses are properly balanced after you write or edit a piece of code. After you get used to paredit, performing — or even watching — manual parenthesis balancing becomes painful.

Recently, I've come across these two introductions to paredit:

1. Emacs Rocks! Episode 14: Paredit
2. The Animated Guide to Paredit

So, if you're still not using paredit, have a look at those and give it a try. At first you might feel like the karate kid doing frustrating chores — you can always take a break with M-x paredit-mode — but I promise it'll soon pay off!