Modern Screen Space Ambient Occlusion Techniques
This bachelor thesis briefly reviews six SSAO techniques. It could be a good start.


CryEngine 2 AO
StarCraft II AO
HBAO
Volumetric Obscurance
Alchemy AO
Unreal Engine 4 AO


http://frederikaalund.com/a-comparative-study-of-screen-space-ambient-occlusion-methods/