Planks on elbows. Keep your back straight and your head down (don't let your lower back dip). Hold for 60 seconds. Read More http://www.details.com/style-advice/the-body/201103/weights-free-workout-the-body-exercise#ixzz2NTYMgtrI
There's a great variety of fitness charts which will be not only excellent addition to your gym, but a good advisor in the performance of exercises as well. You can find fitness posters for leg, abdominal, biceps, forearm workouts and many others.