Scientists have bred tame foxes, and their results may shed some light on how dogs were domesticated. From the article:

They stare you in the face, wag their tails and whine with joy when anyone approaches. But these are not dogs; they are a domesticated breed of fox that looks and behaves just like man's best friend.

After 45 years of selective breeding, and almost as many fox generations, scientists have produced what nature could not, a tame fox who eagerly follows his master's gaze.

...After many generations, and 45,000 foxes later, the scientists noted distinct differences between the selectively bred fox colony and their wild cousins. The foxes also looked different. Their coats developed white patches just like some dogs. Their muzzles became shorter and more puppy-like and in some, the ears became floppy and tails curly.

"Through genetic selection alone, our research group has created a population of tame foxes fundamentally different in temperament and behaviour from their wild forebears," Mr Trut said.

...It was also thought that wolves were relatively easy to tame because they were a pack animal and naturally obeyed orders from those higher in the pecking order. But the latest findings show that foxes, a solitary animal, can also be bred to read the communication gestures of humans.

Here's a related article, including a picture of these tame foxes.