The Humble NordicTrack Skier

If you haven't considered the humble NordicTrack cross-country skier, you should:

• It's a good full-body workout.
• Taku mentions it specifically as a good choice for interval training, and my own experience bears this out.
• There is a balance component to the exercise, and thus really works the lower legs, especially in the first few weeks. It's been great for my shin splints.
• It's dirt cheap. Check out the current eBay listings (make sure you also check the "show only completed listings" box to give you an idea of what these are going for, and note especially all the auctions that end with zero bids!). You can also often pick these up for a song by watching the classifieds or CraigsList. Heck, I've seen them at tag sales or just sitting by the side of the road.

I will warn you though: when you first hop on the thing you'll think it sucks. It'll feel wobbly and awkward. You won't like coordinating your arms and legs, and at least one part of your body will probably fatigue well before the others. So you'll cut your workout short without really feeling like you've achieved much. For me, it took about three weeks to hit my stride on the machine, but now I'm very glad I put in the work to get used to it.