Screen space ambient occlusion is a post-process effect which attempts to approximate shadows produced by nearby objects in a scene. A simple example of this would be darkening of crevasses in objects, as well as the illusion of absence of light found between two objects near each other.
In games, this effect aims to improve the visual realism of scenes by increasing visual depth of objects. The shader works in screen-space meaning it is a post process effect which makes use of (hopefully) already existing per-fragment depth and normal data.
Note: Ambient occlusion is an high-end effect meaning that it has the potential to improve the graphics of your game, however the cost of the effect is still quite high. This will run fine on any PC with a modern graphics card, however caution should be taken when working on slower PCs. As a rough guide, if you can run Ambient Occlusion in other games, then you can almost certainly run this SSAO.
This has currently only been tested on the windows platform, however it should work on both Mac and Linux. Mobile could work, however it is likely that the devices will not be powerful enough to run this effect.
We have a windows demo you can try before you buy: https://dl.dropboxusercontent.com/u/14457523/MarketPlace/SSAO/SSAO_Demo.exe
NOTE: When importing, you will need to change the types of any erroring shaders to HLSL 9, there is a bug with the marketplace imports that default all shaders to GLSL.