Wiktionary

i use this like every day. endlessly useful and fascinating

Tied with Commons as the greatest Wikimedia project. Yes, even Wikipedia is no match. Besides being the most detailed and comprehensive online dictionary out there, it also hosts a whole swathe of words in other languages, which is invaluable to language learners. Do make this your dictionary of choice for English, and if you're bilingual, consider contributing! It's not that difficult once you get the hang of it.

Go on here all the time

The poor man’s dictionary

the etymology sections are always very interesting from a historical perspective. highly recommend browsing by categories like internet slang