Surround helps you manipulate "stuff that surrounds stuff".
Replace the surrounding
<p> tag with a
<div> tag, replace the single-quotes surrounding a string to double-quotes, insert space within the surrounding brackets. As a programmer, it's amazing how often you're dealing with surrounding text, and how much faster the Surround plugin makes it.
Like Match It, this is a plugin that's useful generally, but essential if you edit HTML.