GNU Aspell is a spell-checker which can be used either as a standalone application or embedded in other programs. Its main feature is that it does a much better job of suggesting possible spellings than just about any other spell-checker available 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. (Deb. aspell package description)
Personally, I have nothing against Ispell, despite the above
but the point is that they are great spellcheckers, IMHO, particularly if you do any work with LaTeX.
You can almost certainly remove any unneeded dictionaries (other than your default) however.