Stylify Me offers an easy way to generate a very light style guide. It will show you a site’s background colors, Typography (fonts, sizes, colors and text samples) and a screenshot of the home page. It also shows sample image dimensions, but it looks like it only pulls a few images from the home page, so that’s kind of confusing. Also if your type is white, it doesn’t show since the page background is white.
May be good for very basic use, but it doesn’t replace a detailed style guide by any means.