I'm starting to get into a new sorta sport called slacklining. you take a length of tubular webbing and set it up between to anchor points, the result is a springy half trampoline/half tightrope sort of thing. It helps a lot with your balance and core and center of gravity, It's a blast too, some people can do incredible tricks on them like backflips and stuff, I'm not that good yet, but I'm getting a little better. Discuss all about slacklining and all that here!
P.S. on a better day, I'll take some pictures of the slackline and post them for you to see.