CatBlock is like an ad blocker, but instead of blocking ads it shows you pictures of cats (configurable)
In 2014, Michael Gundlach (the creator of the AdBlock extension) discontinued his pay-monthly "CatBlock" extension, that let users replace ads online with cats (or any other image from Flickr).
As he open-sourced the code, a few contributors to the project decided to fork the code and continue maintenance of the extension.