NHunspell Spell Checking, Hyphenation and Thesaurus
NHunspell is a .NET version/wrapper of the Open Office spell checker Hunspell. It allows to use the Open Office spell checking, hyphenation and thesaurus dictionaries in .NET applications. It is GPL, LGPL and MPL licensed and can be used in close source projects for free.
Go to http://www.maierhofer.de/en/open-source/nhunspell-net-spell-checker.aspx
See also these related projects.