GNU Aspell is a Free and Open Source 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.
