The words "classic" and "legendary" are often overused in the music technology world, but when describing the ubiquitous Shure SM58 microphone, they are more than appropriate. The SM58 can be found in use everywhere from garage band rehearsal spaces, to recording studios, to stage productions.
Shure debuted the SM58 in 1966 as a studio vocal microphone (SM stands for "studio microphone"). However, it quickly became a standard for live use. Its frequency response of 50-15,000 Hz with a peak in the 5000-6000 Hz range makes it ideal for any melodic-type sound -- instruments as well as vocals. Being a dynamic cardioid mic, it does suffer from significant proximity-effect boominess, but a low-frequency roll-off at the board will easily solve the problem.
Connection polarity and pin-outs are standard, with pin 2 positive on the XLR connector. The microphone's impedance is also in the standard range. Its rated impedance is 150 ohms with the working actual impedance being closer to 300 ohms.
Perhaps the most notable feature of the SM58, especially in today's saturated environment of inexpensive and cheaply made stage mics, is its build quality. Few microphones come close to the ruggedness of the SM58. This solid-feeling chunk of metal has garnered legends of using it as a makeshift hammer! A $100 investment in a 58 buys a microphone that will last a lifetime.
In short, the popularity of the SM58 microphone is due to the combination of physical strength, and technical specifications adaptable to a wide variety of situations.