I think, it's good to use Hunspell
Reason for using Hunspell
NHunspell is based on Hunspell and brings the Open Office spell checking, hyphenation and thesaurus to the Microsoft® .NET Framework. NHunspell is a .NET (C#, VB) library and wraps the native libraries Hunspell, Hyphen and MyThes.
The integrated libraries are used in OpenOffice and work with the dictionaries published on OpenOffice.org.
NHunspell is licenced under: GPL/LGPL/MPL. Free use in commercial applications is permitted according to the LGPL and MPL licenses. Your commercial application can link against the NHunspell DLLs.
From one of the comments:
Spell Checker: Hunspell Hunspell is a spell checker,
morphological analyzer and generator. It has stemming functions to find word stems. It allows the generation of derived forms (plural, ...) from the word stem.
SpellEngine - Spell Check,
Hyphenation and Thesaurus for Server and Web Server (ASP.NET) SpellEngine allows the thread-safe execution of spell check requests. It is a component opimzed for servers. All NHunspell features (spell check, hyphenation, thesaurus, analysis and generation) are integrated. To ensure full utilization of the CPUs on several requests, one Hunspell, Hyphen or MyThes is instantiated per processor or processor core and controlled by a semaphore. Multiple dictionaries can be installed to serve different languages. Spell check, hyphenation, and synonyms in multilingual applications are supported without any additional programming.