Since no one has put together a list of online grammar guides/handbooks, I thought I'd make a list of some good ones that are available for free. These guides are not authoritative, but they're from reputable universities or organizations. If you have any reputable sites that you think should be included in this list, let me know and I'll add them.
For an authoritative guide, check out The Chicago Manual of Style, which is the guide used by most publishers. This is not free; you need to either buy a copy of the book or pay for an online subscription.
1. Purdue Online Writing Lab - has pages on grammar, punctuation, citations, and a lot more.
2. Grammar Handbook (University of Illinois at Urbana-Champaign) - has basic grammar rules about parts of speech and has a section of common usage problems.
3. Two classic books from Bartleby.com, an internet publisher of various books (including fiction and poetry).
The Elements of Style by William Strunk - a classic usage guide used in a lot of colleges.
The King's English by H. W. Fowler - another classic guide that contains sections on style, meaning, and euphony.