Chrome bug, got malware or did I did I break it?

I was developing my site when a strange banner appeared. First I thought it was my fault, then that it was the webhost provider fault and now I have no idea of where this comes from.

I know it looks ugly and affects all pages I visit. Look how the footer of StackOverflow looks:

The code itself does not appear in the 'View page source', but it appears when I look at it from the 'Inspect element' and looking at the rendered html. There are more div but some of them are empty. There is a lot of foreign code here. Here is the main one:

<div id="op_ad">
<br>
  <center>
    <iframe src="http://www.games-free-apps.com/games/b.html" style="border:none;width:310px;height:255px;overflow:hidden;" frameborder="0" marginheight="0" marginwidth="0">
    <!-- MORE CODE HERE -->
    </iframe>
  </center>
</div>

And a more detailed screenshot of intrusive code, this time from my website:

So, is this a bug of Chrome, something I broke somehow (I never saw these pieces of code) or malware? Any idea of how can I fix it?

I am using Google Chrome Version 21.0.1180.89 in Ubuntu (native from official page).

EDIT: I started a wiki answer. Please add your hacked extension to the list. I'm going to send an email to Google to see to inform them, but I think they might already be aware.

I just had the same thing - the extension which was broken for me was one called Join Tabs... Dodgy!

I had the same problem and it turned out to be the extension "Web RSS Subscriber" that was to blame.

Apparently some extensions in Google Chrome, includying old and obsolete ones, have been hacked. If you are having the problem stated in the OP, please follow these steps and add the extension that causes the trouble to the list.

Known extensions that have been hacked:

• Inkognito Tab Switch 2.2

• Incognito Switcher

• Join Tabs

• Web RSS Subscriber

Steps to find out the rotten extensions:

• Disable all your extensions. Options->Settings->Extensions

• Enable the first one. Open a webpage where you have had the same problem, and check if it happens now.

• If it happens, add that extension to the list. If it doesn't happen, disable that extension and repeat for the next extension.

I had the same problem with JoinTabs. At first I thought it was just an ad on the page until it started appearing on every webpage. I would suggest JoinTabs (no icon). It serves the same purpose but doesn't have the stupid ad appended to every page:

Link

You will need to drag and drop it into the extension window though since this is outside the Chrome Web Store.