Progress Telerik UI for WinForms
The SpellChecker component notifies users of misspelled words and provides suggestions for corrections. It supports word-by-word as well as all-at-once spellcheck modes.
Another useful feature is the support for custom dictionaries so you can provide your own set of words or language of choice.
You can add as-you-type spell-checking functionality to your textbox controls. Equipped with a large dictionary, RadSpellChecker can identify misspellings and will flag them with the familiar red squiggly underline. By right-clicking on a flagged word, you can choose to replace the word with one of the word suggestions, choose to ignore the flagged word, or add the flagged word to the dictionary.
In addition to built-in integration with RadTextBox and our own text box implementation, RadTextBoxControl, RadSpellChecker has built-in support even for Microsoft TextBox (and its inheritors).