Here are a few links I consult regularly, or find interesting or useful.

• Thefreedictionary.com: One search gives you access to medical, legal, and a number of other authoritative dictionaries as well as thesauri and usage examples. Always my first stop.
• Merriam-Webster, unabridged: In recent years, I find I disagree with the editing choices in M-W‘s collegiate version—why are “face-to-face” and similar constructions hyphenated, alone or after a noun, completely unnecessarily and contrary to long-held guidelines of hyphenation? But the unabridged version is a good, ad-free resource by subscription.
• Chicagomanualofstyle.org: The Chicago Manual of Style online. The home page has links to CMOS’s FAQ, which is fascinating reading, as well as a quick reference to Chicago style.
• Apstylebook.com: The AP Stylebook online. Order the book from the site and browse a number of features.
• Poynter.org: Informative website from a leading organization for journalists.
• Theslot.com: Site of Bill Walsh (R.I.P.), a chief copyeditor for national news at The Washington Post and author of several funny, fantastic books on language and usage.
• Copyeditors Knowledge Base: This site, created by Katherine O’Moore-Klopf, is an encyclopedic resource for editors and those interested in learning more about language or the profession of editing.