Not on line (or maybe they are), but when I supervised the writing of foreign nationals writing across the globe for a worldwide news agency, I sent out these paperback references. And I use them myself when I want a clear, simple, fast explanation:
David Daniels and Barbara Daniels, English Grammar (HarperCollins College Outline series)
Eugene Ehrlich, Punctuation, Capitalization, and Spelling (Schaum's Theorgy and Problems series)
I use the Chicago Manual of Style. Simple, it ain't, but it is thorough.
If I need something explained in terms I can understand, I go to Grammar Girl.
quickanddirtytips.com/grammar-girl
Might note that if you're thinking of springing for a copy of the Chicago Manual of Style, you'll want the latest, 16th edition. This is the first edition with a grammar section in it. The problem with the CMS is that when the specific examples of what to do get sticky, the CMS sometimes just stops giving advice.
You have to have some way of judging the source of the guidance, though. Many folks slapping guidance on the Internet don't seem to feel the necessity to give anything but their untrained opinion. Putting a blog up doesn't make one a grammar/punctuation expert.