AccuSpell

AccuSpell is a small application that, once installed, underlines all misspelled or unrecognized words in red. This works on any part of your computer (browser, notepad, search bars, etc.).

The application includes a number of options for excluding words written in uppercase (like acronyms) and email addresses. It also lets you add any word to the application's dictionary.

AccuSpell is a very useful tool. Unfortunately, though, this trial doesn't include spellcheck in any other languages, so it's mainly intended for users with extensive knowledge of English who want to fix misspellings and other errors.