AdiumX makes use of this service by default!
Safari supports it as well, but it is off by default. To enable Safari to use the spell checker, open a web page that has a text box (i.e., http://www.google.com) put the cursor in the text box field and go to Edit, Spelling and check Check Spelling as You Type. Weird spot for this preference setting, I would have expected to find it in preferences. Once you set it, it will stay on for every web site.
I hope more and more applications choose to make use of the system wide spell checker.