When you write in Word, OpenOffice Writer, or any other high-powered word processor, you've got built-in spell checking, grammar checking, and more. But any more, lots of your writing doesn't take place in one of these desktop apps. Writing blog posts, creating Tweets in Twitter, and lots of other places have you composing directly in your web browser, where these types of tools aren't necessarily available to you.
You can add the ability to check spelling, grammar, and usage with After The Deadline. This tool is available as an Add-on for Firefox. Click in a text field on your online form, hit the After The Deadline hotkey, and it'll take a look a things and let you know what it thinks might need attention, highlighting spelling errors in red, problems with grammar in green, and even style suggestions in blue.
And if you don't happen to have Firefox, they've made the core functionality of this tool available as a bookmarklet that you can use with Safari and Internet Explorer.
After The Deadline is free for personal use. All you'll need to use it is a recent copy of Firefox and a dedication to making (and fixing) spelling errors.