Martha, Grant, and the Way with Words team can only answer so many questions. The "Reference Works" section of this site is great but seems to mostly point to print grammar guides. Typing a question into Google brings up seemingly endless sources – many of which are of questionable value.
So, when you need to have the best possible answer about a grammar question, where do you go online? Particularly, if you need to support your position to someone else on a grammar issue, what are your two or three definitive resources?
As importantly, what are the resources you avoid relying on (even if they support your position)?
When I'm working, and need a definitive answer to a grammar question, my main source is the Center for Writing Studies at University of Illinois at Urbana-Champaign: http://www.cws.illinois.edu/workshop/writers/
When I'm just browsing during free time, I'll often drop by Grammar Girl: http://grammar.quickanddirtytips.com/ Almost always learn something new there, even if I'm not searching for specific answers. Covers grammar and etymology, presented in a light and sometimes humorous style. Unfortunately, over the last year or so, the number of ads on this site has vastly increased.
Note: I tried the "grammar check" feature on MS Word. For about a week. It's been disabled ever since that brief initial trial. The AI engine is not yet ready for prime time.