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.
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)