NOTE: This question was composed in an attempt to follow Guidelines for Great Subjective Questions. I hope I do not fail them too hard, but if you see how the question can be improved please edit it or let us discuss it on chat or meta. Apologies if I failed searching and if this is a duplicate.
There are two sites that I most commonly use as a first stop when looking up words online:
onelook, the reasons are that, for example, looking up a word "dictionary" gives not only quick and short definition (from macmillan), but also links to 44 other dictionaries and to etymonline
wordnik, the reasons for this one are that it gives you immediately on the results page definitions and examples from a few dictionaries that you can compare, however it does not give links directly (compare a lookup for "dictionary" - it gives definitions from four dictionaries, reformatted to consistent look and some examples)
(Both of these are aggregating from different sources, but similiar reasons could be given for specific dictionary sites).
I would like to find out which other dictionaries or sites people are using as the first stop (regardless if they are aggregators or specific dictionaries) and why (in a sense of how or when they would be best used)?
Please one site per answer.