TUAW Tip: Hit esc for some word suggestions

It appears that while typing a word in many OS X apps, such as iChat and Yojimbo, hitting the Esc key will present a drop-down list of word completion options. I *think* this might be a Cocoa-related service of Mac OS X (sorry, Firefox users), but I'm always hesitant to toss out a guess like that because I am constantly surprised as to which apps are actually Cocoa (Finder, for example, is still Carbon, even in Tiger). I get this popup in Ecto, Stickies and Mail as well, but not in any input areas on websites in Safari, so feel free to experiment with this handy but (as far as I know) undocumented feature.
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