AddictSpell has all of these options and many more. Various elements can be shown or hidden as your application requests. All dialogs are also completely replaceable, allowing the application to show its own UI as needed. AddictSpell includes support for main dictionaries, custom dictionaries, and MS-Word custom dictionaries. Any number of dictionary files can be open simultaneously.
Main dictionaries are static lists of words that have been compressed for size and lookup speed.
Though Addict only ships with an American dictionary, many additional main dictionaries are readily available for royalty-free distribution. These dictionaries are formed by the end users of Addict when adding a word or auto-correct entry while performing a spelling check. Addict's configuration system gives your users the ability to both create and modify custom dictionaries. The dialog above is Addict's built-in custom dictionary editor. For even more detailed information on enhancements made to Addict 3 and the differences between versions 2 and 3, see the upgrade information page.