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

• www.thefreedictionary.com: One search gives you access to medical, legal, and a number of other dictionaries as well as usage examples from literature and thesauruses. Once the most complete dictionary site on the Web, it’s now unfortunately slowed by noxious ads.
• Merriam-Webster Unabridged: My replacement for the sad demise in usability in the site above. I often dislike the editing choices of M-W‘s collegiate version–why is “face-to-face” hyphenated, completely unnecessarily and contrary to every rule of punctuation?–but the unabridged is a good, ad-free resource by subscription.
• http://www.chicagomanualofstyle.org: The Chicago Manual of Style online. The home page has quick links to CMOS’s FAQ, which is essential reading, as well as a quick reference to Chicago style.
• http://www.apstylebook.com: The AP Stylebook online. Paid subscriptions are available, but you can also order the book from this site and browse a number of features.
• http://www.poynter.org: Informative Web site from a leading organization for journalists.
• http://www.theslot.com: Site dedicated to copyediting from Bill Walsh, chief copyeditor for national news at The Washington Post and author of Lapsing Into a Comma and The Elephants of Style, some of the best, most cogent books on copyediting I’ve found.
• Copyeditors Knowledge Base: This site, created by editorial freelancer Katherine O’Moore-Klopf, is an encyclopedic resource for editors and those interested in learning more about language or the profession of editing.