Healthiest and/or Most Effective way to build muscle mass without equipment?

I have one dumbbell and plenty of space in my apartment to work with. I used to lift at the gym and never got much bigger, but it’s a bit much to afford right out of school and I’m still used to fitting this sort of thing into a “first real job” schedule.

Ideas for a good work out? I’m currently doing the 200 Squats and 100 Pushups programs (http://hundredpushups.com/) on off nights – squats, crunches while hanging from a bar, and calf raises holding a 35lb dumbell on the first night (all to the 200 Squats program specifics) and pullups, pushups, and chinups on the other night (to the 100 Pushups program specifics).

Something to supplement that with cardio would be good, but building leg muscle (I have REALLY skinny legs) is my biggest concern as I haven’t been able to bike as much as I used to.