One of the most impressive exercises you'll see performed is the one-arm chinup (OAC) or one-arm pullup (OAP). I recently found this excellent discussion of the subject:
One Arm Chinup
There is a longish video (just under 15 minutes) discussing the exercise embedded in the article. It's the meat of the article, so you'll need a good 15 minute block if you want to see it.
It's interesting to hear discussion of injury potential, the difficulty of even gymnasts to perform this, and a very thorough technique discussion. The technique and injury potential is often glossed over - it's usually something like "get 20 pullups, then start working on enough strength to pull yourself up with one arm." This is a bit more detailed, and it's worth considering that you might need to approach attempts at this lift with caution and seriousness.
Please also take a look at the Beastskills One-Arm Chin-Up/Pullup tutorial as well.