CSS Syntax Checker for BBEdit and TextWrangler
John Gruber's CSS Syntax Checker for BBEdit and TextWrangler is a great couple of scripts that will help you keep your CSS as valid as your markup.
I've been a BBEdit user since the mid-nineties, and use the keyboard shortcut to invoke its built-in markup validator so much I'm surprised my keyboard still works.
I've been missing a CSS validator though, and find it a bit of a hassle going to the W3C CSS validator to check my syntax. No more. Great!