If you are short on time and/or space for a workout, I have a good one for you. The workout I just did is your ticket to fitness on even the most cramped day.
I started off with a 5 minute warmup jumping rope. If you don't have the space to jump rope or need to shorten the warmup, go with at least 3 minutes and simply do jumping jacks. After warming up, I simply did variations of the same 3 exercises over and over for 15 minutes.
Push up variations (there are a million)
Curl up variations (million as well)
Lunge Variations (not quite a million but a bunch)
Here is a link to a great set of push up variations and some good information about them courtesy of Men's Health.
Since I only did 3 exercises, I only took a minute break after 2 sets through and 4 sets through, 6 sets through, and so on. Some of the push up variations I did were regular, stagger, T Push ups, and med ball push ups. Some of the curl up variations were regular, curl ups with med ball, bicycle curl ups, and Russian Twists. Lunges included regular, lunge with twist and med ball, side lunges, lunge and returns.
After 15 minutes of the rotations, I finished up with 5 more minutes of jumping rope. Again, you can do this or jumping jacks or walk/jog a few minutes. Whatever fits in your schedule and your space.
All together, I needed next to nothing and I was done in 25 minutes.