Cambridge Dictionaries Online includes examples of usage and mentions differences between British and American usages and spellings.
Dictionary.comAdvantage:Each entry lists definitions from several dictionaries. Tabs for Thesaurus, Encyclopedia, Reference and Web. Good resource for finding a substitute for a phrasal verb, for example: to get a hold of = to obtain
Merriam-Webster.The Gold Standard: Type in your word and decide whether you want the dictionary or the thesaurus.
Yourdictionary.com includes also a section called ”Other Dictionaries,” which includes grammar guides.
ספרים וחוברות הדרכה ברשת
The Chicago Manual of Style. You can't live without it, or at least some of us can't make a living without it!
The Blue Book of Grammar and Punctuation. Free access to some of the sections; nice layout; good review of and easy access to punctuation issues. Page on "effective writing" provides an introduction to the ground rules that will help improve your skills. Also has a page with quick reference explaining those confusing pairs: that vs. which; then vs. than; etc.
The Literary Encyclopedia. The TOC alone tells you everything you need to know to write well. Worth browsing just to distinguish your strengths from your weaknesses. Click on the asterisk to open the link.