This week BKeeney Software released a new plugin for Windows and Mac OS X that lets Xojo developers do cross platform spell checking. Mac OS X developers have the option of using the System Spell Checker or Hunspell. Windows 8 developers can use the System Spell Checker or use Hunspell. Pre-Windows 8 developers can use Hunspell.
Hunspell is a popular spell checker used by many popular open source applications and is the basis for the Mac OS X spell checker. Free Hunspell dictionaries for most languages are available.
More information on pricing, demo plugin, and more is available at http://www.bkeeney.com/allproducts/bks-spell-checker-plugin-for-xojo/