I’ve recently been trying out Coda - a one window web development application for Mac OS X, and so far I am impressed.
It combines a text editor, CSS editor, web browser and FTP all within the same application. And if that doesn’t sell it to you other features include a HTML validator, a place to store ‘clips’ of code and even built in reference guides for HTML, CSS, Javascript and PHP.
It’s looking like a definite purchase when the free trial runs out.