Indoor Rowing Machine.

Awesome, really.

It mimics the behaviour of a row boat but using a fan that creates the necessary resistance with the flow of air. This is particularly interesting as even when you adjust the resistance of the cable the actual resistance depends on how you row: The faster the more resistance in the same way that in a boat the faster you row the more effort you will need to travel against your bow wave. 

There is also a selector for the cable resistance but this doesn't make much difference (at least I haven't noticed).

You can use this baby for a lot of things, one of them a hell of a cardio workout, just try to keep your strokes per minute between 24-30 spm. 

I use it for a 10 minute warmup before lifting.

It recruits legs, butt, arms, shoulders, abs, back... in the category of no-impact cardio it is only second to the elliptical. 

It is also a beloved piece of equipment in CrossFit and you can use it very effectively for HIIT.

Just one note of caution: It is addictive!!!