Grammarly - Clear, effective, mistake-free writing everywhere you type.
Glosbe - Huge collaborative massively multilingual dictionary.
Hunspell - Hunspell is the spell checker of LibreOffice, OpenOffice.
Visuwords - The fun and free way to look up definitions. THE graphical dictionary and visual thesaurus.
DSpellCheck - Yet Another Notepad++ Spell-checking Plug-In
Wordnik - the biggest online English dictionary, by number of words