What would be well-known, and in a certain measure reliable, online resources that should be checked before asking a question, or that could help with answering questions?
I've got a few listed that I use frequently, but I'm sure it is far to be exhaustive and maybe not the best in their field for some.
Dictionary
Etymology
Idioms
Fundamentally the real question about the acceptability or authority of some dictionary definition is “who is doing the lexicography?”
The most respected organizations doing lexicographical research are well-known groups like Merriam-Webster, Oxford Dictionaries and American Heritage. These are organizations which have been making dictionaries for decades (if not centuries) and employ trained linguists and lexicographers. Sites like Dictionary.com or The Free Dictionary are just an aggregator-distributors of lexicographic data created by others. So a citation from The Free Dictionary is not in and of itself acceptable or not acceptable, but rather the acceptability is judged by ultimate source of the data. A search for the word idiom on TheFreeDictionary.com gives results from American Heritage and Collins dictionaries, which are pretty reliable. Wiktionary can also have good data but it should be taken with a grain of salt. Their lexicographic methods are inconsistent: some of it is as good as any other dictionary. Sometimes it harbors undisclosed biases, and sometimes it is just amateurish.
Most of the people answering will probably know the basic meanings of the words already; I think looking them up is more for the asker's benefit, to see them, share them, and establish a starting place for people to help. I think for this purpose, the dictionary that is easiest for them to use would be acceptable, as long as it is not grossly inaccurate. When the bar is higher, for more complicated or difficult cases, I think OED wouldn't get objections, but it's not freely accessible. I personally like Merriam-Webster and Wiktionary, which has the benefit linking to multiple languages.
I disagree with who says it's a subjective matter.
Or better I think it can be so to a certain point. If I want a serious definition and I can access to any resource in a library (for example) I will not just look for any dictionary but rather look for the better ones, same goes for other situations, such as the one related to this site.
Someone who studies languages is aware this, being more familiar with dictionaries and language studying.
I tend to use the Oxford English Dictionary, which usually provides good resources about Etymology, but I have found myself using also the Oxford Advanced Learner's Dictionary too. I think those two are the most known among English learners.
The corpora (Contemporary American English, Historical American English, British National and others) at BYU have useful tools. The page linked also conveniently provides links to the much larger but less reliable Google corpora.
I find myself occasionally using the Middle English Dictionary, especially when Etymonline is being unnecessarily cryptic.
My opinion: there are no "common accepted online resources". Somebody may like one resource and somebody else may dislike it. It's a subjective matter. Even more, let's say we talk about dictionaries. One dictionary can have clearer definitions, other better etymology, yet other one a down-to-earth pronunciation guide.
I don't think this website should endorse one website or other. It's also a matter of fairness-there are many websites out there about the language and new ones emerge rapidly.
However, if somebody asks for some specific resource, (s)he can be helped but we keep in mind that it is a subjective, personal, suggestion.
I'd suggest World Wide Words as a useful resource. See here... http://www.worldwidewords.org/ particularly because when I forget to look on it before posting a question, I am quite often referred to it.
I like The Free Dictionary. It has a number of technical dictionaries as well as standard written English and some other languages.
