Check out
www.thundersplace.com and decide for yourself. It's a free message board that is run on donations.
The basic concept is that you force blood through your penis using your hands, and the increased pressure causes microtears in the penis chambers, and they heal back larger. Then there is also hanging and stretching, which basically just stretches or extends the ligaments.
The main exercises you hear about are:
-Jelqs (basically milking your semi-erect cock with an OK grip)
-Manual Stretches (just grabbing and pulling)
-Hanging (specialized thing that attaches to your dick, and then you hang weights from it)
-Pumping (yup, using those pumps that you see in the back of porn mags)
I was skeptical at first. Have a look at thunders: there is no way people would write bullshit posts, routines, pictures, etc for 3 years. + evidence in porn. (A lot of people who use Penis Englargement techniques end up with slighly bruised/beat up dicks)
Gains vary, 3-4" in one month are bullshit. It's more like 1 inch of length a year, .5" girth after working hard for one hour a day everyday, consistently. Some guys put in many hours a day.
Check out the size database:
http://www.thundersplace.com/pedata/data.php
Believe.