Speckie, which, if you're completely devoid of higher brain function and need a helping hand, is a concatenation of spell check IE.
Speckie is a free download, and you'll have to restart IE9 before it can take effect. You'll also be asked to approve Speckie when you start IE9 (this seems to be a new security feature in IE9). Once it's up and running, however, Speckie uses almost zero resources, causes no discernible slowdown, and has almost zero configuration. You can turn it on and off and change the current dictionary by right clicking a text box; that's it. There's quite a large variety of dictionaries available on the Speckie site.
In terms of actual spell checking, Speckie is good, but not perfect. Simple text boxes work well, with red squigglies appearing in real time. We had less luck with WordPress and Blogger post editors: sometimes red squigglies appeared, and sometimes they didn't. Moving your cursor to a word always triggered a spell check, though. The right-click menu seems to offer good and sensible suggestions, too.