When the weather is bad, or you don't feel safe running or walking outside, treadmills are great. The only thing that's bad is that you are doing the same repetition of motions over and over again. You use all the same muscles, which is very unlike moving on actual terrain outdoors (and very boring too).
Check out this omnidirectional treadmill. A clever little Michigan-based company called Virtual Space Devices, Inc., has been working on this since 1996. It's a self-contained unit that allows you to walk or run in any direction you choose, without actually going anywhere. It will allow you to move with freedom, and say goodbye to the repetition of basic treadmills.
This could also be used for virtual video games, so kids (and adults) can get off the couch and be part of the game (while burning calories too). I'm not sure this will be hitting gyms or living rooms anytime soon, but just the thought of it is exciting. Check out the video - it's truly amazing.