Ah, the Ergometer!
Check out http://www.concept2.com/us/products/...js=0&id=rowers
If your gym has one, it is probably by Concept2. They make the ones the Olympic Team trains on.
They are a seriously hard-core workout, but you control how hard it is by how hard and fast you pull. In the past, I could do an hour-plus on one, and I definitely was in shape!
(ObDisclaimer: I don't work for Concept2, heh! Or own stock in the company hehehe!)