Why am I seeing ads on websites including Wikipedia?

I was reading about hash functions on Wikipedia when this ad popped up. I was appalled and flabbergasted and annoyed to find advertisements akin to this one dotting my article like sprinkles on a cupcake.

Annoying popups appear after you hover over them.

I now notice that these ads are appearing on any web page I visit including Superuser.

Why? How can I get rid of them?

By the way, these ads have the very gall to surface on Superuser, as seen here:

This is usually caused by "malicious" extensions, and removing the extension will normally solve the problem.

It looks like someone downloaded a "free game" that came with some scummy advertising stuff. Just uninstalling the plugin won't get rid of everything:

• Here is Gamevance.com's official information on deactivating and uninstalling their software.
• Here are some suggestions from a secondary source, including files and folders and registry settings to remove.

Luckily, from the sounds of it, they're pretty good about actually uninstalling when you uninstall.

In case the official instructions don't work, or even if they do, you might want to run an adware removal tool such as Ad-Aware. For every piece of malware that asserts its existence you are likely to have a dozen or so that do not.