We've just wrapped another of our grabbable widgets - this one's to promote I AM LEGEND, and will be available as an automatic update for everybody who grabbed our BEOWULF widget.
Personally I hate the word 'widget', but it's rapidly becoming part of the lexicon of online marketing to describe these kind of embeddable applications. If you want one of the earliest examples of this kind of thing, and a strong indicator of how successful it can be, the proliferation of Youtube videos in their early days was driven by precisely this principle. In an age when everybody has their own little piece of screen estate somewhere on the web, all you have to do is give them something cool or useful enough and they'll do your advertising for you.