Online Proofreader by Grammarly is a browser extension that will proofread writing online: in email, social media, Google Docs, etc. It will also detect quoted text and generate appropriate citations.
Grammar Girl Podcast: Each episode is about a different grammar or style topic and teaches you about writing in a fun and friendly manner, with “a quick and dirty tip, a meaty middle, and a final tidbit.”
The 30 Most Common Grammar Mistakes (and How to Avoid Them): Learn to fix or avoid some of the most common mistakes.
Guide to Grammar and Writing: This site has detailed and easy to understand explanations for a huge variety of grammatical concepts plus interactive quizzes to test your knowledge.
Grammar Bytes: This is a free and online, self-paced course focusing on grammar, usage, and mechanics.
Common Errors in English: This site explains common errors in English, including commonly confused words (such as to/too/two) and other confusing aspects of English grammar.
Online Editor by GrammarCheck will check a block of text for spelling, grammar, and punctuation issues and suggest corrections.
Elements of Style: Online version of Strunk & White’s classic style guide.