Merriam-Webster Collegiate (2003)

I don't recall how I stumbled upon this online version of the M-W Collegiate; it's an old interface, but it beats the pants off their new web version, with its clunky design, ads, clickbait, and assorted hoo-ha. Enjoy it while it lasts.

OED

This link is to the Public Library of Cincinnati and Hamilton County's OED page, and a Cincinnati library card number and PIN are required for access. If you're not a card-carrying Cincinnati library patron, check with your local library to see if they have an OED subscription available to you.

Freedictionary.com

The Farlex Free Dictionary is, as I understand it, an online version of the American Heritage Dictionary of the English Language, Fifth Edition. Thesaurus included. It's a good resource, if the site design is a bit of a yard sale.

Webster's 1913

Though old, this dictionary gets a fair amount of praise. It looks like a fun browse, but it's probably not ideal for everyday use. I've not spent much time with it, but have added it here for future reference.