Coda supports context sensitive help on their Help books documentation modes which works on double clicking of functions and entities in any supported language. As you do your word processing, Coda offers spell checking and can search for text across several files within your site.
There is subversion support, so now with Coda you can check out code, update, commit and change on supported systems. In addition to this, there is a feature called Open Quickly which allows Coda to open files without typing the entire file name.
Coda allows developers to set up projects called sites, create and edit files within the sites, and preview the sites in the internal view base on Apple's Webkit. The application uploads the files to the web server but not before debugging. All this can be done in one window.