Now that I've been going to the gym a lot, and old idea (by old, I mean back in 1992 Physics class) that I've always wanted to see happen is starting to happen:

Energy produced by workouts being converted to electricity!

It's still fairly inefficient, but I will say that if I knew I could generate electricity for my house I'd be on the bike a lot more often (mostly to try and make up the cost).  But at a gym- incentive programs could be produced that would help offset the equipment cost.  For example, my gym offers an incentive program for using more of their resources.  Their system could be easily adapted to harness something like this- points for how many watt/hours you produce, for example. I predict that seeing a tangible item such as that would be immensely popular and encourage people to use the equipment more often.

Here's an article that talks more about it... we'll see if more gyms start doing this soon.

Okay...

 if you ever needed proof that there are people out there that ... well, just watch this because you already know what I'm going to say...