another good variation, is the towel pullup, I love these, I use 2 small hand towels draped over a pullup bar chalk up the hands and get pulling, fantastic for hand strength. You can also do these with pieces of rope, go to your hardware store buy 2 small lengths of thick rope and drape over the bar before like with the towel pullups, much more difficult to grip!
I sometimes wrap towels etc over the bars as well for extra thickness (if you dont have access to a thicker bar!).
Pullups, chinups etc are so versatile and easily one of the most beneficial upper body exercises.