Currently in rotation, I have noticed a number of advertisements that are embedded frames. While I don't mind banner advertisements, and firefox prevents the majority of pop-up and pop-under advertisements, this recent type actually overlays itself on the page, preventing the view or accessing of the information under the ad unless a refresh is done (and the random chance that you won't get another such ad.)

So far the only one I can recall offhand is one for heavy.com, though there have been others.

I just thought I would bring this to your attention.