It looks like you CAN get on to the website for the Challenge for free (http://www.self.com/challenge) so as far as I can tell, you don't need a subscription to participate. If you don't know what it is, its basically a three month program with three components: cardio, strength and eating. The cardio component is really flexible- whatever you like to do. So you can walk every day, run a couple of days, or do an exercise video a few days a week.
The strength component is a series of exercises that you do in succession- none of them are particularly difficult, but when you put it all together it is a great, fast strength training workout. I remember that my muscles really started to look more toned by the end of the first month.
The eating component is great- lots of flexibility, lots of healthy, fresh food, and easy recipes. If you follow the portions they tell you, it's hard to feel hungry, plus the combination of produce, grain and meat that they use (it's 50% produce, 25% grain, 25% protein) is super filling.