P90X

P90x is the fitness craze of the summer, but it has been building a following for years.  This is a fairly simple dvd set that you can pop in and use to workout at home.  Very few weights are required so you don't have to put in too big of an upfront cost in order to do this routine, which is part of the reason it is so popular.  

The routines consist of about an hour each and you do six days per week of workouts.  Some of these will be active recovery type of days, though even the yoga is a pretty good workout.  While they don't have you lifting big weights, you will do pullups, pushups and lunges til the cows come home, and your muscles will definitely be sore.  I'd doubt that someone who stuck with this routine and a good diet (don't be fooled, you don't have to do their diet) would not be fitter by the end of 90 days.