- The dictionary should be monolingual. (I've made a similar thread for bilingual dictionaries.)
- List a single site you think is best, not a list of different sites and not a link to such a list. You should be able to search for a word directly from the site you link to. (It's probably the link you're most likely to use yourself if you want to look up a word online.)
- If someone else has already added your language and you think a different site is better, add that one too!
- Keep my list in your message and your own to it (so ideally by the end of the thread we'll have a long list in one post).
I'm starting with a shorter list for this one because really the only languages I use monolingual dictionaries regularly in are Spanish and Estonian.
Diccionario de la lengua española (Real Academia Española)
Eesti keele seletav sõnaraamat (Eesti Keele Instituut)