Simple and easy to use. Free and doesn't require registration.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
Gramlee.com - Online grammar check, proofreading and copy editing services.
Stylewriter - StyleWriter edits any document.
GrammarChecker.net - Grammar Checker is an online spell check tool. It effectively works on English sentence correction, Punctuation mistakes of 28 other different languages.