They are cute adorbale and often they cost nothing. Mixed-breed dogs are the subject of this article and yes, you may be cynical enough to say that it is apparently just an excuse to put loads of really cute dog pics on the internet. And yes, you would be right.
From the article
The humble mutt is the product of two dogs of unknown breed - even though they can sometimes be fairly accurately guessed. Dogs will do it like they do it on the Discovery Channel with anything else that remotely resembles another dog. Often they get it right, though sometimes the human leg is used as a stand in. It is only when there is a massive disparity in size that interbreeding is impossible. That means the mutt comes in a huge variety of shapes, color and size and often they can defy physical classification.