Great collection of useful sites from the PRSA to help us all become better writers.

Dictionary.com — A reputable site that licenses Random House Webster’s Unabridged Dictionary and The American Heritage Dictionary.

Longman Dictionary of Contemporary English — An excellent source for distinguishing meanings of American and British English words.

@JohnEMcIntyre Twitter feed — A Baltimore Sun copyeditor for more than 23 years, John McIntyre is the dean of copyeditors.

@Copyediting Twitter feed — A must for in-depth insights on the latest language issues.

Style & Substance blog — The Wall Street Journal’s language blog by longtime Journal copyeditor Paul Martin.

Grammarphobia blog — Written by renowned authors and former New York Times editors, Grammarphobia is one of the best language blogs available.

The Chicago Manual of Style — The online version is only available by subscription, but a Q-and-A section is available free.

Other online resources to expand your English vocabulary and grammar:

Enso Words universal spellcheck

Orangoo free spellcheck

Verbix: conjugate verbs in 100+ languages

WordCounter

Visuwords online graphical dictionary

Merriam-Webster’s Visual Dictionary

OneLook
This site searches hundreds of online dictionaries, encyclopedias and other references to find the word you are looking for.  Type a concept, phrase or question into the search box and OneLook will return a list of words and phrases related to that concept.  You can also find a specific word if you know its definition.

Definr — the incredibly fast dictionary
You can also add the Definr search extension to your Firefox browser for easy dictionary access and subscribe to the Definr Word of the Day RSS feed with your newsreader to expand your English vocabulary.

source: PRSA