Thankfully there are many tools—with more arriving all the time—that automatically generate styleguides based directly upon the code you write. Recently I was tasked with creating a living styleguide by my employer and I decided to take a good look at my alternatives before starting. Following is a run down of the strengths and weaknesses of various options. While I wasn't able to investigate all of them, I think I covered a diverse group.
It became clear in my research that there are two kinds of generators: some are better for designing while others are better for documenting. I've split them in to these two groups.
Generators for Designing
If you're still trying things out, exploring, or designing in the browser these are the tools for you.
Pattern Lab is built by Dave Olson and Brad Frost based upon Frost's atomic design methodology. Atomic Design is a fantastic concept, starting out with that level of organization is big win. Pattern Lab comes out of the box with a pretty complete set of HTML elements to ensure you don't forget anything when creating your base styles. That way when you actually need an HTML5 Range input down the line, you're already covered.
If you don't like the idea of working with PHP there is also a Node version, but it's apparently behind in terms of features. Brad Frost has also documented his typical Gruntfile for compiling Sass files in conjunction with Pattern lab.
Style Prototype is built by Mason Wendell and Sam Richard of Team Sass and just might be the Cadillac of styleguide generators. Style Prototype projects are scaffolded with Yeoman and include a wide arrary of features including (but not limited to) automatically generated Style Tiles, versioning for style, deployment to a preview server, and exportation of your finished product as a Compass extension.
You will need to be pretty familiar with Sass as the .scss files—while intelligently organized—are a little tricky to wrap your head around at first. Style Prototype is probably the most fully featured of the generators, but it is also the most opinionated in terms of workflow and structure.
Apparently 2.0 is on the way, so keep a lookout.
Generators for Documenting
If you're working with existing designs, sites, or patterns consider one of these:
Kalei, KSS, and Styledocco
Kalei, KSS, and StyleDocco are all pretty similar. They parse your preprocessed or plain old stylesheets for formatted documentation blocks and spit out a documentation site with a default style. These are definitely the quickest options for getting you up and running with a styleguide. Check The Style Guide Guide for their various technology stack compatibilities.
Because of it's simplicity, Hologram is easy to integrate in to your project or workflow. You can see how I've integrated Hologram in to my grunt workflow on github.
I ended up using Hologram because of my task at hand (documenting a design that was mostly complete) and my tendency as a developer to want the ability to customize, but all of the options have serious strengths. I suggest you take at least two for a spin before heading too far down one path as you may discover features that are missing or undesirable. Hopefully this comparison helps to point you in the right direction. Good luck!