I've written favourably about the new gov.uk site before, and this week I see they've also published their style guide and content guidelines.
For comparison, here's the house style and plain English guide I helped write for my present employers. The Economist, Guardian and BBC style guides are also useful reference points, particularly for anyone coming in to an organisation that hasn't yet developed its own rules.
This latest addition is an impressive bit of work: most of it accords with what we're already doing in our publications, and while there a few points I would quibble with (they advise against writing out numbers below 10, for instance) there are certainly a few more we could incorporate (eg on SEO).