I went through my Editing > Style Guides bookmarks folder, here's what I found:
The Yahoo! Style Guide, despite having bit of a! silly! name!, is an easy-to-understand resource, particularly for non-writers. I recommended to a non-profit client I have; they seem to like it. (One caveat: It focuses as much on coding and interface as it does on language.)
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.)