Here at PRPL we've crafted quite a few sites that include an admin section or intranet of some kind. A few weeks ago I was browsing around Github, and found this really great project Web-App-Theme.

The project is described as:

A simple layout by Andrea Franz that you can use in your web applications. Inspired by cool themes like Lighthouse, Basecamp, RadiantCMS and others, it wants to be an idea to start developing a complete web application layout.

This project is really well done, it's got a myriad of pre-built ui items like tabs, sidebar navigation, tables, feedback messages, etc. all created in a way that makes it trivial to extend and change the look completely. There are about half a dozen contributed themes ready to use, within a few hours I had a customized theme to match the rest of the site I was working on.

Along the way I found a few Internet Explorer 6 bugs (crazy right?), added a few bells and whistles, and modified some things to match our standards here. Overall It sped up development time quite a bit and I will definitely use it again in the future.