I was just about to ask this same question. I've found a bunch of resources and a few solid examples, most of which have already been referenced in other comments:

http://rizzo.lonelyplanet.com/styleguide/design-elements/colourshttp://ianfeather.co.uk/a-maintainable-style-guide/http://kaleistyleguide.com/http://pea.rs/https://github.com/styleguidehttp://warpspire.com/kss/http://codeguide.co/http://sfdc-styleguide.herokuapp.com/http://brettjankord.com/projects/style-guide-boilerplate/https://ux.mailchimp.com/patterns/https://www.mozilla.org/en-US/styleguide/websites/sandstone/http://paulrobertlloyd.com/about/styleguide/http://24ways.org/2011/front-end-style-guides/http://www.codecademy.com/blog/141-codecademy-reimaginedhttps://medium.com/ui-designs/e5bb62db91e5http://style.codeforamerica.org/https://www.mapbox.com/base/http://www.yelp.com/styleguidehttp://demo.patternlab.io/http://daverupert.com/2013/04/responsive-deliverables/http://semantic-ui.com/http://resource.github.io/fabricator/http://bem.info/method/http://voiceandtone.com/https://docs.google.com/a/groupswitch.com/spreadsheet/ccc?key=0AiN0QfBTPpOCdDFjWlM0eU1ra21XanZkekxGbjA2WWc#gid=0http://www.mojotech.com/toolkithttps://github.com/astanush/ux/blob/master/style-guides.md#frontend-style-guideshttp://disqus.com/pages/style-guide/http://chriscoyier.github.io/SurveyMonkey-Design-Patterns/http://vinspee.me/style-guide-guide/http://stuffandnonsense.co.uk/blog/about/an-extract-from-designing-atoms-and-elementshttp://trulia.github.io/hologram/http://sourcejs.com/http://livingstyleguide.org/