LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
AntiPlagiarist - AntiPlagiarist checks multiple documents for plagiarism.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
iThenticate - Prevent Plagiarism in Published Works
WhiteSmoke - WhiteSmoke software products for writing and language. WhiteSmoke is a grammar checker, spell checker, style checker and more - in a single package. Also featuring translation software and English video courses.
PlagScan - The Internet makes plagiarism a bigger problem than ever.