American Heritage
The American Heritage Dictionary of the English Language, Fifth Edition. The AHD recently replaced Merriam-Webster as my everyday dictionary. Unlike M-W, the AHD is not afraid to weigh in occasionally with useful guidance on current-day standard and nonstandard English usage. It's a sensible, practical approach, useful to anyone whose daily work involves making editorial decisions.
Garner's Modern English Usage
This is not an online source, but it's important, so it goes on the list. At the moment, I don't own or use the most current edition of Garner but the previous edition, Garner's Modern American Usage, published in 2009. If you're springing for one, get the new one.
Freedictionary.com
The Farlex Free Dictionary includes the American Heritage Dictionary among its sources. It's a useful site, offering a number of tools, though the design is a bit of a yard sale.
Oxford English Dictionary
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.
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. The interface is basic, as is its search intelligence. I've not spent much time with it but am adding it here for future reference.
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 its layout is clear, easy to use, and much more satisfyingly dictionary-like than their current web version.