How much do we float in atmosphere? The atmosphere is a fluid and we have volume, therefore we displace some of it and some buoyancy force must exist. How strong is it? How much does it affect gravitational acceleration on the surface of Earth? And considering only buoyancy (no drag), how much difference would make falling in atmosphere and a vacuum?
 A: You will displace air equal to your volume.
If your volume is $v$,you will displace air of volume $v$,
If the density of air is $d$,
then the mass of the air displaced will be equal to the product of volume and density.
and 
thus force on you according to Archimedes ' principle =$vdg$,
'g' is the acceleration due to gravity.
Archimedes' principle indicates that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.
http://en.m.wikipedia.org/wiki/Archimedes%27_principle
A: Average human body volume = 0.0664 $m^3$ (seems low to me, but that's according to Wolfram Alpha: http://www.wolframalpha.com/input/?i=volume+human+body).  Density of air depends on temperature and pressure, but is about 1.2 or 1.3 $kg/m^3$.  That means we displace, on average, about 80 g of air, giving us a buoyancy of about 0.8 N (about 1/6 lb).
The acceleration due to gravity and buoyancy would be $a = g - \frac{F_\text{buoy}}{m}$.  To take round numbers, use $m = 80 kg$.  Then the second term is $-0.01 m/s^2$, reducing the acceleration by about one part in 1000.  (The effect of drag becomes much bigger pretty quickly as you fall.)
A: Not much. By Archimedes principle the buoyant force is equal to the weight of the volume of fluid displaced, which would be the volume of your body. And the fluid is air. So you would only be slightly heavier than you are now if there was a vacuum at the earth's surface.
