Hanging windshield wipers
Hanging windshield wipers (or "Windscreen wipers" as I prefer) are a great exercise for your obliques, and what's better they're easy to do yet still look pretty cool!
Because you only need an overhand grip technique and don't push yourself up and over the bar, you'll also find this is something you can do in most gyms without having to get your hands on a good calisthenics frame.
Start by hanging from the bar using an overhand grip with your hands shoulder length apart. Next, you will want to pull your legs up (feet together) so they are parallel to your hanging arms. You're back should be parallel to the floor, that way your arms and back should be at a right angle to each other and your back and legs should be at a right angle to each other.
Now keeping your form together so that your arms remain straight and your back remains parallel to the floor, rotate your torso so that your legs rotate 90 degrees to the left and are parallel to the floor. This is your starting position.
What you now want to do is rotate your torso 180 degrees so that your legs go from being parallel to the floor pointing to the left, to parallel to the floor pointing right. If done correctly your legs and torso should perform a full wiping motion. That's one rep.
The trick to making windscreen wipers look good is to keep a strict form. Most people should find the wipe action doesn't have a hard impact on their legs, but keeping your back parallel to the floor puts a strain on your shoulders, so having good shoulder strength will help you with this technique.