I've been using a Total Gym since 1997 (I'm on my second one, which I bought in January 2008), the machine popularized in the infomercials with Chuck Norris and Christy Brinkley. It uses your own body weight as resistance and can be set to various levels of resistance. What I like about this machine is that you can go from exercise to exercise by simply changing the position of your arms on the fly, without missing a beat. This allows me to do five or six exercises, one right after the other, with no rest in between, which is superior to any circuit training routine you could do in a gym, where you have to wait to get on the next machine, or walk across the room to get to it. In short, I can get in a full-body workout of all my muscle groups in about five minutes. At this pace, you also get an aerobic effect. This, plus walking is all the exercise I need.

http://es.youtube.com/watch?v=Kcz0MLLizeY

The model shown in the video is the old Total Gym 2000, which was the first one I had. I used it for ten years.

I OWN ONE.ITS GATHERING DUST IN MY BASEMENT

We have one at work, ours goes up to level 10 though.  It is the commercial model and is not able to be folded.  You can get a good workout on it, but I rarely use it personally.  We use it mostly for pateints to do squats and heel lifts before they are able to fully weight bear.