Following on from a discussion on Handakte WebLAWg on style, here are some style guides:
For the USA:
The Chicago Manual of Style
The Chicago Style Manual website also has FAQs, for example on how to write Internet and Web terms, although I prefer email to e-mail
For the UK:
The Oxford Style Manual (this consists of the Oxford Guide to Style and The Oxford Dictionary for Writers and Editors in one volume, which may be a bit unwieldy – I haven’t seen it).
The EU has an online English style guide.
BBC News Style Guide
(click on Open Guide 1)
Sweet & Maxwell (British legal publisher) House Style Guide.
There are some links about style in the original Joi Ito entry of January 18th. Note that the Chicago Manual of Style is more about formal requirements, whereas Strunk and White’s The Elements of Style (mentioned in a comment) is about writing style.
Then there is writing in a way that makes sense on the Web. Jakob Nielsen is a good source there.