If you're doing any JavaScript development, use an advanced editor that highlights errors and warnings as you type. This kind of functionality has until recently been the province of static language editors only, but the free NetBeans 6 achieves this amazing feat. Traditionally, you first discovered JavaScript typos and simple bugs when first loading the parent page into a browser. The speed-up you get from being able to short-cut these iterations is profound.
NetBeans 6 also highlights CSS errors and is a more than capable editor for most all languages a web developer is likely to use these days.
