A spelling checker
http://aspell.net/

GNU Aspell is a spell checker designed to eventually replace Ispell. It can
either be used as a library or as an independent spell checker. Its main
feature is that it does a much better job of coming up with possible
suggestions than just about any other spell checker out there for the
English language, including Ispell and Microsoft Word. It also has many
other technical enhancements over Ispell such as using shared memory for
dictionaries and intelligently handling personal dictionaries when more
than one Aspell process is open at once.