This question came up in one of my conversations with a friend.  What is the mass of light?  Well, there are a couple of equations relating to energy and light.  The most famous was developed by Albert Einstein: E = mc², where E is energy, m is the rest mass, and c is the speed of light in a vacuum (3.0 x 10⁸ m/sec)
Another very famous equation is Planck's relation: E=hc/y, where E is energy, h is Planck's constant (6.626 x 10^-34 J s), c is the speed of light in a vacuum (3.0 x 10⁸ m/sec), and y is the wavelength of light.
So putting these equations together gives the following: E = mc² = hc/y.  Thus m = h/cy.  The wavelength of visible light is from 400 to 700 nm.  So to calculate the rest mass of light in the visible range, that has a wavelength of let's say 500 nm, we plug in the information and get the following:
m = 6.626 x 10^-34 J s/ (3.0 x 10⁸ m/s x 500 x 10^-9 m)
m = 4.417 x 10^-36 kg
So there you have it.  The rest mass of a single photon of light at a wavelength of 500nm is 4.417 x 10^-36 kg.  Since light is never at rest, this number is actually the "effective" mass of a photon of light.
Another quick calculation is the momentum of a photon of light.  Momentum is defined by the following equation: p = mv where p is the momentum, m is the mass, v is the velocity.  Using the above mass for light at 500nm wavelength, and plugging the numbers into the momentum equation gives:
p = (4.417 x 10^-36 kg) x (3.0 x 10⁸ m/s)
p = 1.325 x 10^-27 kg m/s
So the effective momentum of light is 1.325 x 10^-27 kg m/s.

To confirm this hypothesis, the mass of light must be measured experimentally.  To measure the mass of light, first obtain two "perfect" mirrors (mirrors with 100% reflection) and weigh them.  Then bounce light between the mirrors and weigh the mirrors while the light is bouncing back and forth between them.  The mass of the light trapped between the mirrors is the difference in weight of the mirrors before and after the light was trapped between them. This method will unfortunately not work since the mass of light is so small and "perfect" mirrors are be attainable.
Another method to measure the mass of light that is probably more realistic would be to take a flashlight of known mass.  Then in a vacuum at zero gravity, turn on the flashlight.  The momentum of the light leaving the flashlight will push the flashlight in the opposite direction.  Then after a period of time, the velocity of the flashlight can be measured, and the momentum of the flashlight can be calculated and from this the mass of light can be calculated.