Grammarly - Clear, effective, mistake-free writing everywhere you type.
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.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
DSpellCheck - Yet Another Notepad++ Spell-checking Plug-In
Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
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.