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:
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.