I'm told that the Oxford English Dictionary (OED) contains around 600,000 entries. While nothing can surpass the OED as an authoritative resource, the website OneLook.com references 18,955,870 words indexed from 1061 dictionaries.
This can be especially helpful for students working on poems and other composition exercises.
Additionally, there is even a "Reverse Dictionary" feature which can help you find the mot juste simply by describing a concept and then it returns a list of related words.
Examples of OneLook.com searches
bluebird Find definitions of bluebird
blue* Find words and phrases that start with blue
*bird Find words and phrases that end with bird
bl????rd Find words that start with bl, end with rd, with 4 letters in between
bl*:snow Find words that start with bl and have a meaning related to snow
*:snow or :snow Find words related to snow
*:winter sport Find words related to the concept winter sport
**winter** Find phrases that contain the word winter
expand:nasa Find phrases that spell out n.a.s.a.