p90x. But I dont strictly follow their schedule, I usually do it 3-4 times a week. You can do half of their workouts anywhere, and the other half with a pullup bar and some free weights.
I've been doing P90X for about 2 months now (entering phase 3 this week). Before P90X, I did almost zero physical activity - I didn't play sports in high school and in college the best I did was ultimate frisbee maybe once a month. I haven't been following the diet plan which is probably a mistake but I've seen fairly steady weight loss and muscle gain - for the first time in my life I can do pullups (I'm up to 2!) and pushups! It's an intense program and will (absolutely will) wear you out if you're out of shape but my fitness is soaring and being able to keep up with a game of bball or frisbee is extraordinarily gratifying.
I'm currently doing the P90X lean program. After I finish that, I'll double my weights and will switch to P90X classic. I workout every day, since its pretty easy to make the time commitment when you have a daily routine.
You definitely see results fairly quickly + if you stick to the entire weekly regiment you'll focus on muscles that you tended to ignore before. i.e. for the first time ever I'm getting decent muscles on my legs.
As far as visible improvements(besides actual muscles), when I started out I was doing 10 more or less crappy pullups, and now I can do 15 proper ones(you know where you fully extend your arms)