I guess it's not very common so I'll explain it. You find a cliff near a river / lake (must be deep) and jump off it.
Anyhow, Sunday, my friends Taylor, Justin, Zach and I went to Mirror Lake State Park (Wisconsin Dells, WI) and pumped off a few cliffs along one of it's drainage rivers. The first cliff I jumped from was about 20 feet above the water. After that I jumped off one around 60 feet. I didn't get injured, except when my feet impacted the water it hurt like hell, of course. After that, I jumped off the 20 foot one again more towards the shoreline (the bottom drops of around 3 feet past the cliffs) and hit the bottom, nearly breaking my left ankle. Luckily, I came away with a bit of lost blood, a hell of a lot of pain, and two hugeass scratches. Of course, freefall feels SO great that I'd do it all again (when my ankle stops hurting).
EDIT -
Note, I wrote this a few days ago on another forum, and my ankle doesn't hurt too much anymore.