Speckie, a real-time spell checker for Internet Explorer
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.
Speckie seems to have one particularly annoying bug where it interprets a line break (Enter) as 'disregard my typos on that line.' Maybe that's by design, but we're not sure. It would be OK if spell checking actually occurred in real time all the time, but in practice, when we were experimenting with WordPress, squigglies would appear just moments before our Enter key summarily removed them.
Still, at the end of the day, some spell checking is better than none -- and considering Speckie's minimal resource use and its completely free price tag, there is no reason to not install it.
It's worth noting that Speckie works for all modern versions (we use the term loosely) of IE: 6 through 9. We can only recommend, however, that you use it with the recently-released Internet Explorer 9.
Download Speckie Internet Explorer spell checker for 32- and 64-bit Windows 7, Vista and XP