My favorite piece of gym equipment has got to be the TRX®. It was developed by a Navy SEAL, who wanted a way to work out without weights or machines while traveling. The result is a multi-functional suspension strap that lets you hit every major muscle group, while engaging your core the entire time. It's a little intimidating to look at but once you learn a few moves, you'll be hooked.
And no I am not a paid spokesperson. I wish!
I have a few favorite moves that I put in my workout rotation and am always on the lookout for new ones. The possibilities are endless on this thing.
I seriously think it's underutilized at most gyms I go to. When they were first installed at my local gym, people used the anchor bars to do pull ups, completely ignoring the whole point of the set-up.