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 suggesting possible replacements for a misspelled word than just about any other spell checker out there for the English language. Unlike Ispell, Aspell can also easily check documents in UTF-8 without having to use a special dictionary. Aspell will also do its best to respect the current locale setting. Other advantages over Ispell include support for using multiple dictionaries at once and intelligently handling personal dictionaries when more than one Aspell process is open at once.
The latest version of Aspell can always be found at http://aspell.net
The Suggestion Intelligence is based on a small test kernel of misspelled/correct word pairs. Go to http://aspell.net/test for more info and how you can help contribute to the test kernel. The current scores for Aspell are 88 in fast mode, 93 in normal mode, and 98 in bad spellers mode: for more information about the various suggestion modes Notes on the Different Suggestion Modes.
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For a detailed description of how Aspell differs from Ispell, See Differences From Ispell.