In the same domain as The Chicago Manual of Style (users working "with magazines, newsletters, corporate reports, proposals, electronic publications, Web sites, and other nonbook or nonprint documents"), you also have: The Economist Style Guide Clarity of writing usually follows clarity of thought.
So think what you want to say, then say it as simply as possible.
Keep in mind George Orwell's six elementary rules ("Politics and the English Language", 1946): Never use a metaphor, simile or other figure of speech which you are used to seeing in print.
Never use a long word where a short one will do.
If it is possible to cut out a word, always cut it out.
Never use the passive where you can use the active.
Never use a foreign phrase, a scientific word or a jargon word if you can think of an everyday English equivalent.
Break any of these rules sooner than say anything outright barbarous.
english.stackexchange.com/users/110/VonC
The Grauniad , Observer and guardian.co.uk style guide .
"No passion in the world is equal to the passion to alter someone else's draft." - HG Wells
english.stackexchange.com/users/72/Jon Hadley
For someone doing technical writing for the US Gov't, I have to recommend NASA SP-7084 : Grammar, Punctuation, andCapitalization: A Handbook for Technical Writers and Editors .
english.stackexchange.com/users/513/Dori
There are a few out there
but some tend to be industry specific.
The best general, free and online style guide that I know of
is the Wikipedia: Manual of Style .
Here are some others: The BBC News Style Guide , Apple Publications Style Guide and Yahoo!
Style Guide (this one is pretty good).
english.stackexchange.com/users/4776/Alex Trueman
I went through my Editing > Style Guides bookmarks folder, here's what I found: As VonC mentioned, the Economist Style Guide is both well-regarded and easy to understand.
The print version is purported to be more thorough than the online, free version.
(I haven't used it much yet, but it looks to be sound after taking a quick dip in the site.)
Onlinestylebooks.com lets you search through a boatload of style manuals.
(Some results seem to link to subscription sites, but many do not.)
The links to the Chicago Q&A columns and AP's Ask the Editor feature are outside their paywalls.
Jack Lynch's Guide to Grammar and Style is a good reference (although I disagree with a few things in it).
In addition to traditional style guides, You can also refer people to pages from some of the excellent grammar blogs out there: Grammar Girl is the only one I know of with a large enough backlog to search through, although others might be able to add to this.
Slate also has good grammar and style columns from time to time, like this piece on why one should not type two spaces after a period.
Also, keep in mind that many older books about style and grammar are now available online, for example, the 1906 Chicago Manual of Style .
(Use with caution; I'm not finding early versions of Chicago or AP there, and surprisingly, I also don't see the 1926 edition of Fowler's Dictionary of Modern English Usage there.)
english.stackexchange.com/users/161/Goodbye Stack Exchange
You can find 88 style guides listed at OnlineStylebooks.com , 70 of which are indexed for search.
OnlineStylebooks.com is owned and maintained by Mary Beth Protomastro, the founder of Copyediting newsletter, the copy chief of More magazine and former editor of the Time magazine stylebook.
OnlineStylebooks.com is not affiliated with any of those publications.
Mary Beth created OnlineStylebooks.com to help copy editors (including herself) quickly consult a variety of style guides.
If you know of a manual that’s on the Internet but not on OnlineStylebooks.com, please tell Mary Beth!
english.stackexchange.com/users/9001/Hugo
For British English, these are widely used and respected: The Guardian and Observer Style Guide
The Economist Style Guide BBC News Style Guide Fowler's Modern English Usage (sadly only the pocket version of the updated text seems to be available online) For matters of online style, I feel that it often comes down to issues of usability, so I refer to Jakob Nielsen's articles , because they're based on solid research.
(Example: " Writing Hyperlinks: Salient, Descriptive, Start with Keyword ")
For the philosophy and reasoning behind style guides, David Foster Wallace's essay "Tense Present" is unbeatable.
english.stackexchange.com/users/94867/A E
I don't like prefixing all my answers and pontifications about English usage with IMHO.  Where can I find a reasonably well recognized style guide that is online that I can refer people to?  My first choice,  chicagomanualofstyle.org , is for paid subscribers only.  (Emphasis on well recognized, personally, I mostly do technical and office writing)
Where can I find a reasonably well recognized free style guide that is online?
english.stackexchange.com
<style-manuals>
english.stackexchange.com/questions/442
english.stackexchange.com/users/6/MatthewMartin
There are a number of well-recognised style guides which you may consider referring to. These include The Economist Style Guide, the Guardian and Observer Style Guide or Jack Lynch's Guide to Grammar. There are many style guides or manuals that can advise you, and George Orwell has written 6 useful rules for any writer: 
Keep in mind George Orwell's six elementary rules for writing good English:  
Never use a metaphor, simile or other figure of speech which you are used to seeing in print.  
Never use a long word where a short one will do.  
If it is possible to cut out a word, always cut it out.  
Never use the passive where you can use the active.  
Never use a foreign phrase, a scientific word or a jargon word if you can think of an everyday English equivalent.  
Break any of these rules sooner than say anything outright barbarous.    
There are a number of well-recognised style guides which you may consider referring to. These include The Economist Style Guide, the Guardian and Observer Style Guide or Jack Lynch's Guide to Grammar. There are several style books available online including: BBC News Style Guide, Apple Publications Style Guide, Yahoo, The Guardian, The Economist Style Guide and Observer Style Guide. Onlinestylebooks.com  lets you search through multiple style manuals.  In addition to traditional style guides, you can also refer to some of the excellent  grammar blogs  out there e.g. Grammar Girl. Also, keep in mind that many older books about style and grammar are now available online.
Jack Lynch's Guide to Grammar and Style, Grammar Girl, The Chicago Manual of Style, OnlineStylebooks.com, The Guardian and Observer Style Guide, The Economist Style Guide,  BBC News Style Guide,   Fowler's Modern English Usage are all suggestions for style guides. There is a huge range of free style guides available online as well as grammar blogs. Some prominent examples include The Economist Style Guide, The Guardian and Observer Style Guide or the BBC News Style Guide.
There are various style guides that can advise you, and George Orwell has written 6 useful rules for any writer. Many editors will try to change your text because they feel compelled to rather than because it is incorrect. There are many style manuals available online. There are many guides for this such as   The Guardian and Observer Style Guide  
There are several style books available online including: BBC News Style Guide, Apple Publications Style Guide, Yahoo, The Guardian, The Economist Style Guide and Observer Style Guide. Onlinestylebooks.com  lets you search through multiple style manuals.  In addition to traditional style guides, you can also refer to some of the excellent  grammar blogs  out there e.g. Grammar Girl. Also, keep in mind that many older books about style and grammar are now available online.
There is a huge range of free style guides available online as well as grammar blogs. Some prominent examples include The Economist Style Guide, The Guardian and Observer Style Guide or the BBC News Style Guide.