There are loads of chin-up variations that are easier or more challenging, depending on what you want, but with a kneeling chin-up, you get both. It's easier on the shoulders at the bottom position, and it's harder because it reinforces a full range of motion.
As the name suggests, you start the chin-up from a kneeling position. Ben Bruno, a personal trainer whose clients include Chelsea Handler and Kate Upton, explains that if your shoulder bugs you from a dead hang, you can still get full extension in a kneeling position without stressing your shoulders out.
Beyond ensuring a full range of motion, I've noticed that this variation really eliminates, or at least drastically minimizes, swinging and kipping, which are pet peeves of mine. I guess you could call these "Anti Kipping Pull-ups."
If you want to make kneeling chin-ups more challenging, you can start the exercise from a dead stop. For this to work though, you either need to be really tall or use a shorter pull-up bar, which you can set up in a smith machine or a squat rack.
Kneeling Chin-Ups [Ben Bruno]