Rowing is outstanding. I've been doing it for a while as a cardio routine to offset my random spurts of shin splints.
When looking for another cardio routine, there was just none that did it for me. Elipticals are a weird motion, and they just don't seem to make me tired or feel like I'm actually working out. The same for the bike/spinning. I just didn't feel tired after or that I had accomplished much.
Then a family memeber got me on to rowing. My God, what a great machine. Not only does it work your legs, but it works your upper body as well. All parts, all while doing cardio. It is fantastic, and I fully reccommend it. I'm used to running, but this kicks my ass as well. You are actually accomplishing a ton while doing it, and you feel great after.