I'm getting advertisements on Craigslist and Wikipedia?

[some guy s]

This is kind of weird. Basically, when I go to Craigslist it's usually straight forward and simple. But I noticed an advertisement (like Adsense) on the top of the page today. Not Craigslist posts, I mean the ads that you click on and it brings you to a different site. This one in particular was for T-Mobile and Android. I also went to Wikipedia and noticed the same thing, only this one was square.

I have never known either of these sites to use ads for generating revenue, so it seems a little suspicious to me that they both do it at the same time. Also, I'm frequently getting the word "Code" written where an ad should be (like on YouTube, Facebook, etc.), instead of the ad. Any known viruses or anything that would cause this? Thanks!