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.
source: 1, 2