Many people, myself included, feel that for all its advantages, the internet has precipitated a steady decline in the quality of writing. Anyone can write anything anywhere, and while that gives a voice to many who otherwise might not have a public forum to share what they have to say, it also makes it difficult and sometimes impossible to uphold any sort of standards.
That said, the internet also offers plenty of resources for improving your writing. Here are a few of my favorites:
Here you'll also find a thesaurus and several other reference tools. It may not be the Oxford English Dictionary, but it gives you plenty of good definitions and sometimes includes usage notes with practical implications for your writing, like differences in how similar words are typically used.
Difference Between http://www.differencebetween.com
Speaking of differences, this is a really cool site. As its name suggests, it's all about the differences between things. It discusses classic word matchups, like affect vs. effect and among vs. between, but its scope extends much further, covering everything from existentialism vs. nihilism, to cataract vs. glaucoma, to pumps vs. platform shoes.
Google searches using quotation marks
If you're trying to figure out how a certain word is used in context or gain a better understanding of the subtleties of a phrase, go to Google and put quotes around a phrase you're thinking of using. Look through the results for reputable sources' use of the phrase or even put quotes around the name of a source in your search. If you're trying to decide which is the better of two phrases, run quoted searches on both and compare the results. For examples in scholarly writing, use Google Scholar.
Grammar Girl on QuickAndDirtyTips.com http://www.quickanddirtytips.com/grammar-girl
Grammar Girl answers questions, provides thorough explanations on common grammar snags in plain English, and illustrates her points with straightforward examples. She also has a podcast on iTunes, though I've never listened to it.
Purdue University Online Writing Lab (OWL) https://owl.english.purdue.edu/owl/
This site has concise tips on everything from coming up with ideas to grammar and punctuation. I think its best feature is its information and examples for different citation styles. The dirty little secret teachers usually don't admit is that, in practice, citation conventions aren't as standardized as they're made out to be. I have found, as have many people I know, that you can have five different teachers who swear by five different interpretations of one rule or another. As long as you're consistent and respect teachers' pet peeves, you'll usually be fine.
Son of Citation Machine http://citationmachine.net/index2.php This site is a citation generator. You put in the requested information for the work you are trying to cite and select your desired citation style, and it comes up with a citation. You have to be careful and double check it, though. As we all know from Spell Check, machines have their limitations.
This site has bilingual dictionaries, language forums, and other tools and references. I'm a big fan of the language forums and have found this site particularly helpful for ESL/ESOL students.
For more online writing resources, check out this article on Open Education Database http://oedb.org/ilibrarian/150-writing-resources/
Two "Offline" Writing Resources:
"Find" and "Find and Replace" (Control + F on PCs, Command + F on Apples)
Use these functions to edit your writing. For instance, before submitting anything I write, I open "Find" and hit the space bar twice to make sure I don't have any extra spaces between words.
The Elements of Style by William Strunk, Jr. and E.B. White
There's a reason English teachers everywhere swear by this classic writing style guide. It's comprehensive, clear, full of examples, and only about 100 pages. A great book to have with you whenever you write.