Windows, Mac, or Linux
To use the spell checker with the Mozilla Firefox browser or the Mozilla Thunderbird email client, you'll need to install an addon from addons.mozilla.org.
You can also use the spell checker directly within LibreOffice by installing this addon. Our thesaurus and hyphenation software come together with the spell checker in one convenient package. If you prefer OpenOffice.org, the same package should work but you might want to download it from the OpenOffice.org extensions site, here.
If you'd like to check document within Microsoft Word, check out these packages sold by the company Cruinneog. The word list in Cruinneog's version of GaelSpell is identical to the ones in the addons linked above.
There is another version of GaelSpell for Mac OS X (10.6+) now available from snasta.com. The name of the package is MacLitriú, and it works seamlessly with Safari, iWork, TextEdit, iChat, srl.
I also distribute source packages for the standard open source spell checkers ispell and aspell.