There's a rather good selection at http://www.bbc.co.uk/dna/h2g2/classic/A935570#Grammar : On-line Dictionaries, Thesauri, Style Guides, etc \ Grammar, Punctuation, Style etc [bbc.co.uk]

Personally one of the best writing books I have ever found is called "The Elements of Style" by Strunk and White. This is a classic on basic grammar and writing skills. It is short, direct, and packed with great info. I was introduced to this book in college and have kept it all these years and even replaced it when my college copy wore out.

I just bought "The Chicago Manual of Style" about a
week ago and have been thoroughly impressed with it.
I had 3 question and it was simple enough to find
the answers. Well organized with good examples. On
the negative side, it is a bit pricey, about $50
Canadian.

*** "The Elements of Style" by Strunk and White

*** The Chicago Manual of Style

I think those two pretty much nail it for a US audience.

There are several other less well known ".... Manual of Style" titles for other audiences.

I forget the name of the one that Oxford? or Cambridge? Press did a while ago.

The Oxford Dictionaries online offering - Ask Oxford [askoxford.com] - can be a useful resource for those seeking clarity in issues relating to grammar.

Not only that, but like all good reference materials, it's a jolly good read in its own right.

Syzygy

The Chicago Manual online.
Garner's Modern American Usage.
Roget's Thesaurus.
OneLook Dictionary.
Microsoft Manual of Style.
Writing Good Sentences by Claude W. Faulkner.