A Sturmey Archer 2 speed "kickback" hub has just been introduced. They are in stock at the wholesaler and we can supply.  They will cost £70 plus fitting, and are an ideal low maintenance solution for riding in mostly flat places. In use  it will work in the same way as the Duomatic described below.
 
The only other two speed made in recent years was the Sachs Duomatic. It was controlled by the chain. If you pedaled back a bit, then forwards, it would change from one gear to the other. If you pedalled back a bit more, then the coaster brake would come on.