Advertising is an unfortunately necessary part of life - and I say that as a guy whose livelihood depends upon it. But there's a fine line between an acceptable level of distraction and an infuriating invasion of a user's space. Case in point: sellAring. Instead of placing banner ads in free apps or even embedding them in the user's status bar, sellAring places audio ads (remember those, radio listeners?) over the "ring ring" sound you hear when calling another party.