Worst case - Those would be the ones on I had encountered free(ish) sites like Sourceforge and OpenClipArt.org (though OCA I think has been fighting them) Thos

Worst case - Those would be the ones on I had encountered free(ish) sites like Sourceforge and OpenClipArt.org (though OCA I think has been fighting them) Those are the ads that try to fool you into thinking their ad as if it is part of the sites UI "Click here to download app!" Download viewer to open file. They put a big black spot on the unsuspecting site that gets those ads. Secondary would be the unscrupulous JavaScript ads that some sites get by signing up to an ad network - such ads that bring up malicious (or seemingly malicious) popups pop-unders, etc. Also putting a bad rep on probably a not as attentive but honest blog.