Almost as quickly as first reported, Google has been quick to take down two extensions that were serving adware to users. Add to Feedly and Tweet this Page are the two named extensions, both were popular and useful extensions in their initial incarnations, but after a while both began serving pop-ups to their clientele.
It seems that Add to Feedly's reason for the change is a change in ownership, the developer in a blog post has described how he was offered "a four-figure offer for something that had taken an hour to create" and like most people probably would, he decided to take the money, a decision the developer now regrets.
Shortly after the sale went through, Amit Agarwal noted that the new developers pushed an update that surreptitiously added advertising features to the extension, ads that were effectively invisible, replacing links on websites you visit and just in general including adware into every page you visit.
Since this is the first time this has come to the forefront, other news sites such as Ars Technica and OMG Chrome are suspecting that the rabbit hole may go even deeper than these two extensions, but thankfully it appears that Google are already taking a hard line.