LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
SSuite Spell Checker - SSuite Office's free spell checker provides you with a handy means of checking the spelling in order to make sure your words are correctly typed or written.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
DSpellCheck - Yet Another Notepad++ Spell-checking Plug-In
Gramlee.com - Online grammar check, proofreading and copy editing services.
tinySpell - A spell-checker for Windows. Checks text in clipboard and/or on the fly in any Windows program. Small and easy to use. Resides in the taskbar notification area. Free download.