Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
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.
GNU Aspell - GNU Aspell, usually called just Aspell, is a free software spell checker designed to replace Ispell.
DSpellCheck - Yet Another Notepad++ Spell-checking Plug-In
Enchant - The easiest way to scale personalized customer support
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.