Bison and domestic cattle can hybridize, and they produce viable offspring. This cross is called Beefalo and is also a pretty popular meat. Technically, a "true" beefalo is 3/8 bison, 5/8 domestic bovine, but most people consider 1/2 and 1/2 crosses to be beefalo as well. They are more laid back and considered to be easier to handle on the farm/ranch than pure bison. They retain many of the benefits (lower fat and so forth) of the pure bison as well. They also tend to be less prone to diseases than either of the parents.
There are actually very few bison herds still in existance that are considered to be free of any domestic bovine genetics, the herd of Plains Bison (Bison bison bison) in Yellowstone being one of them. There is also a herd of the slightly larger subspecies, the Wood Bison (Bison bison athabascae), in Canada that is considered "pure".
More info on Beefalo can be found here:
http://americanbeefalo.org/ 
