GNU Aspell is described as ', 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'. There are five alternatives to GNU Aspell, 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 Hunspell or ispell. Other great sites and apps similar to GNU Aspell are myspell (Free, Open Source) and Enchant (Free, Open Source).
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.