Here's a body weight exercise for those who are looking for challenges. I have a love-hate relationship with pull-ups. It's very rewarding when you start building up strength and manage to do few of these in a row. But before getting there you are going to face some frustrating moments when you just hang in there and literally can't pull yourself up.
I like to practice pull-ups with a partner who can hold my feet on those last repetitions when I need an extra push. You can also use an elastic exercise band that takes some of weight off and makes the whole exercise easier. Another good way to work on your strength is to hold yourself up for a while and and then let yourself down slowly.