I know it is almost cliché to talk about quick fix workouts for those with limited time, but I am a firm believer that in order to obtain and maintain a fit and stellar physique, there is certainly no need to be be killing yourself for hours on end at the gym. Along with the aforementioned Tabata Method, this is definitely one of my favorite no frills, no hassle workouts, and often used in my rotation. Basically, it is based on Ross Emanait's "No Excuses" system, and all it takes is 10 minutes per day.
In a nutshell, you choose four calisthenics exercises that you can do comfortably, yet are still somewhat challenging. I typically like push ups, burpees, squats, pistols, handstands, pull ups, and muscle ups, but you can also do running in place, box jumps, planks, mountain climbers, shadow boxing, etc. You should preferably have enough variation to make for a full body workout, so try to include both the upper and lower body. These four exercises shall be performed in this manner, with no breaks:
Round 1: Each exercise 1 min = 4 mins
Round 2: Each exercise 45 secs = 3 mins
Round 3: Each exercise 30 secs = 2 mins
Round 4: Each exercise 15 secs = 1 min
Total elapsed time = 10 mins
And there you have it! 10 minutes to a lean, mean physique using nothing but bodyweight exercises. Oh, and did I mention that you do this all nonstop? Of course, this would be the ideal, but if you need to catch your breath or if you get stuck on an exercise, by all means, take a quick break, but just make sure it's not too long! And as mentioned earlier, don't pick movements that are way too easy, nor ones that will have you gasping for air in a matter of seconds!
An alternate pattern I like to use is this:
Round 1: Each exercise 1 min = 4 mins
Round 2: Each exercise 1 min = 4 mins
Round 3: Each exercise 30 secs = 2 mins
Total elapsed time = 10 mins
This one sometimes works better than the first because in Round 4, 15 seconds apiece is sometimes not enough time, considering that it usually takes a few seconds to switch from exercise to exercise.
I normally like to perform this first thing in the morning as the perfect way to kick off the day. Start off doing this 3 times per week, or if you are game, you can even go every single day as long as you switch things up. Prepare to get psyched up physically and mentally for work or school every morning!