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, where you'll need to sign in with your Cincinnati library card and PIN. If you don't live in Cinci, check with your local library to see if they have a subscription available to patrons.

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.