Hunspell is described as 'is the spell checker of LibreOffice, OpenOffice.org, Mozilla Firefox, Google Chrome, and it is also used by proprietary software packages, like Mac OS X and InDesign'. There are five alternatives to Hunspell, not only websites but also apps for a variety of platforms, including Linux, Windows, Mac and iPhone. The best alternative is Druide Antidote. It's not free, so if you're looking for a free alternative, you could try GNU Aspell or ispell. Other great sites and apps similar to Hunspell are myspell (Free, Open Source) and Enchant (Free, Open Source).
GNU Aspell, usually called just Aspell, is a free software spell checker designed to replace Ispell. It is the standard spell checker for the GNU software system. It also compiles for other Unix-like operating systems and Windows.
Ispell is a program that helps you to correct spelling and typographical errors in a file. When presented with a word that is not in the dictionary, ispell attempts to find near misses that might include the word you meant.