Merriam-Webster Online
This is a resource I use many, many times every day. Of course the site includes a free basic dictionary, complete with pronunciation help. If you’re not sure how to spell a word, you can take a stab at it and the search feature will give you a list of possibilities. You can also access a thesaurus, unabridged dictionary, medical dictionary, Spanish-English and French-English dictionaries, and more. (Some of these may be premium services that require a subscription, but access to the basic services is free.) Then there is the word of the day, word games, Word Central for kids…I could go on and on.
Be warned, if you’re a word person (and what writer isn’t?), you can easily get sidetracked and spend an hour of writing time browsing around this site. I speak from experience.