1.) Tool: Reverso
It's a tool for free online translation in French, Spanish, Italian, German, Russian, Portuguese, Hebrew, Japanese, English. Why not Google Translate? Well, Reverso supports translation in context of words, groups of words and idioms and I think that this feature is the most important: Google Translate doesn't provide such amount of examples.
2.) Tool: OneLook
OneLook is not only a site where you can find tons of dictionaries: there's also a tool that can help you to find words related to the one you specify, ranked by relevance.
3.) Tool: Readability-score
"Measure and improve the readability and marketing content of your writing with our exclusive readability and keyword tools!"
I found this tool to be extremly helpful: it can analyze your text snippet or your entire blog article and give you Flesch-Kincaid reading ease, Gunning-Fog score, keyword density, word counts, syllable counts and many other valuable metrics that can help you to improve your writing:
Secret tip: clear your browsers' cookies if the site says that your limit of free analysis is exceeded.
4.) Blog: dailywritingtips
"The rise of the information age – with the proliferation of emails, blogs and social networks – makes the ability to write clear, correct English more important than ever."
This blog contains lots of good articles: Top 10 Punctuation Mistakes, Pronoun Mistakes: Which Is Not for People, Verb Mistakes: Mixed Errors.
5.) Blog: caroclarke
Blog by Caro Clarke where you can always find good writing advice, for example, in the beginner's four faults article.
8.) Bonus: words and vocabularies by theme