Google posted on the AdSense Google+ channel that the ad balancer tool which is a tool that can be used to show less lower paying ads on your site, is not back up and running.
Google wrote:

We are happy to inform you that the Ad balance tool is back and you can now use it again to help you reduce the number of ads you show to users, specifically the ads that earn you the least revenue, and see how it affects your earnings.

I had no idea it broke – to be honest.
Here is what it looks like:
Here is a screen shot of the tool:

You can drag and click apply to adjust your ad balance:

Forum discussion at Google+.
Update: Now when I try it, it says it is under maintenance…