My same gal that brought her pushup numbers from 20 to 50 without us doing pushups(though we do a fair amount of ring dips) has had a tremendous improvement in her grip as a result of having to hold on to the pullup bar, the kettlebells and other things that try to come out of her hands. She had a friend in school that trained in fencing with her. They had this little post training contest where they would test each other's grip by shaking hands and squeezing. Until recently, this was always a stand off. Kelly said that she ran into her fencing pal recently and they had their little contest and when she squeezed Kelly dropped her friend right to her knees where she was yelling "Owww" and telling Kelly to stop.
It's amazing what we are getting better at without realizing it is happening and without making a specific effort to train it. Next time someone asks you what you are building when you are doing swings or pullups, tell them "My grip."