Here are a few great tutorials on this subject:

5 Simple, Effective Exercises For Workaholics With No Time To Spare

8 Simple Exercises For Busy Bodies To Get Your Heart Pumping

3 Fast, Simple 10 Minute Exercises That Can Reshape Your Whole Body

Tricep Dips.

Sit on the edge of a chair or bench with your hands next to your thighs. Lift your body out just in front of the chair, with your feet flat on the floor and your knees bent. Lower yourself down so your elbows bend no more than 90 degrees and push yourself back up.

• Start off with a few small sets and build up from there.
• Straightening your legs increases the workout.
• Avoid this exercise if you have wrist or shoulder problems.

Pushups.

If you’re in great shape, do 100 correctly formed pushups as quickly as possible, taking breaks as needed.

If you struggle to do one good push up, try doing them on your knees, or stand at the base of the stairs and put your hands on the stairs. The key is to make it easy enough that you can do at least 10-20 repetitions.

Here are a few more ideas:

• You can do pushups with your hands wider than shoulder width to get more of a shoulder and chest workout.
  
  
• If you bring your hands together into a diamond shape near the center of your chest you will work more of the triceps and shoulders.
  
  
• Working half pushups - either going only half way down and then back up, or starting from the floor and only pushing up half way and then going back down - will intensify your pushup workout.

Squats.

Body weight squats are a great exercise you can do anywhere and a fantastic way to learn proper technique. Start off your squat workouts by lowering yourself only a foot or so. Then work up to deeper squats as your muscles become accustomed to the exercise.

• If you're feeling a bit advanced and in need of a more intense workout, grab two 2-liter bottles filled with sand and try performing squats while holding one in each hand.

Bodyweight squats. From a standing position, squat down and stand back up. Do as many as you can and try to go all the way down until your thighs are parallel to the floor. If that’s too difficult, grab a doorframe to pull yourself back up.

Jump squats. These are similar to the bodyweight squats, but instead of just standing up, you’re going to jump as high as you can, land softly, and repeat. Do as many as you can. This is a tough one.

Crunches.

Crunches are great for your abs if you do them correctly. Done incorrectly, they can put undue stress on your neck and back. To avoid injury, be sure to use the correct techniques.

• The biggest risk is if you jerk your neck to give you the force to crunch. While doing crunches, keep your hands behind your head and your elbows back while using your stomach muscles to lift your head and chin towards the ceiling.
  
  
• Once you've mastered this technique, try lifting your legs off the ground or assuming a bicycle position with one leg bent and the other straight off of the ground.

Calf Raises.

Calf raises can be done wherever there is a raised surface, such as a stairway. If you need balance, hold onto the railing. Facing the stairs, stand with only your toes on the stair and your feet extending out past it. Standing straight up, lower your body down below the level of the stair and then back up onto your toes.

• Even though it seems as if this is an ankle exercise, it also works out your calf muscles.
  
  
• The biggest emphasis on this exercise is the ability to go from an extreme negative position with ankle below toes to an extreme positive position, ankle fully raised.

Jump rope.

If you haven't tried jumping rope lately, now is the time to give this fun and invigorating exercise a try. Purchase a jump rope or fashion your own from any sturdy piece of rope. Start slowly and gradually increase both your speed and intensity. This heart-healthy exercise can be performed a few times a day for maximum exercise benefits.

The first time I tried it, I could only last about 30 seconds before getting completely winded. It's definitely a workout.