Pressure of electromagnetic radiation

Electromagnetic radiation exerts pressure to everything that absorbs or reflects it. If 100% of EM radiation reflected then it would have twice as much pressure as it would have with 100% absorption.
This pressure of radiation (if all energy was absorbed and not reflected) in pascals is its power in watts per square meter divided by speed of light. Average solar radiation power per square meter is about 1370 W per square meter and causes about 4,6 micro pascals of pressure (like weight of 0,46 milligrams per square meter or twice that if all that reflected). 1 pascal is like 100 grams with earths gravity per 1 square meter and normal atmospheric pressure is about 10 tons per square meter.
This small pressure from sun is enough to make Nichols radiometers rotate. These use paddles with mirrors on one side for one way rotation and near vacuum environment to reduce air drag.

This pressure has been used to accelerate ions and small pieces of foil with powerful lasers. Circularly polarized light cause more even pressure. The laser power mentioned in this study had intensity of 10¹⁷−10²² W per square centimeters. They reached particle energies of up to 200 million electronvolts while in comparison about 10 000 electronvolts of energy is needed to create gamma ray photons. Very thin pieces of foil could accelerate in one piece.

1 watt laser can keep 5-10 micrometer particles hovering and with focus this laser could push with force that was 55 times more than the force from gravity on these particles.

Weak light pressure has been used to achieve coldest temperatures measured. In laser cooling devices light keep atoms suspended in middle of containers away from container walls that could transmit heat to atoms. With these devices temperature of around 1 billionth of a degree above absolute 0 have been achieved.


Pressure of radiation from hot objects

Stefan-Boltzmann law (source with calculator to find these values) describes among other things how many watts of power (P) per square meter (A) exit object with some temperature (T). Sigma value is constant. Increase in temperature increases power of heat radiation exponentially. 10 time difference in temperature causes 10 000 fold difference in energy of heat radiation. With higher power of heat radiation comes also faster cooling as heat exit it faster. Reflective objects emit less powerful radiation but this formula can give idea about how much energy could exit object in ideal circumstances.
Some results i got with previously mentioned formulas for 1 square meter area in all cases (reflective surfaces could have up to 2 times more pressure from these radiations):

In room temperature it radiates ~450 watts and causes pressure of ~1 micropascals.
100 C object could radiate 1090 watts and causes 3,6 micropascals of pressure.  
800 K wood fire emits about 23 000 watts and causes 75 micropascals of pressure. 
5800 K on surface of sun emits 64 megawatts per square meter (0,02 pascals).
150 000 K degree object radiates enough heat to cause pressure of 1 atmosphere (100 000 pascals).
50 000 000 degree nuclear explosion emits ~3,5×10²³per square meter with pressure of ~1×10¹⁵ pascals that is comparable to 10 billion atmospheric pressures. That's like 100 billion tons or 100 cubic kilometers of water on 1 square meter of earth (if gravity was even for entire column). That pressure is enough to start nuclear fusion in thermonuclear weapons. 
Tevatron particle accelerator creates up to ~1 trillion degree heat (Large Hadron Collider has achieved higher temperatures). If square meter could be heated to this temperature then the heat radiation could cause pressure of ~1,6×10³²pascals which is ~1,6×10²⁷ times atmospheric pressure. It's like 1,6×10³¹ kg in Earths gravity per square meter. Mass of Earth is ~5.9736×10²⁴ kg and mass of sun is ~2×10³⁰ kg so even mass of sun on 1 square meter may not be enough to overcome the pressure from 1 square meter 1 trillion degree heat.