Aptana is an Eclipse plugin that came to me highly recommended by a peer (Doug Sims) and has proven itself to be a real assett to me as well. What does it do? Only provide code assist with both Javascript and CSS files! Another VERY COOL thing it does within the code assist is provide visual indicators of which CSS and Javascript attributes are IE and/or Firefox compatible, something that is a constant plague to those trying to maintain cross-browser compatibility.

Visit Aptana.com and click the "Download Aptana Plugin" link on the right hand side of the page for instructions on how to install.

One caveat of this plugin is that it's code assist feature is only available when working with .JS or .CSS files...inline css or javascript, well, you're on your own.