Indirect Illumination : The thing that makes global illumination a challange!

What are the methods/techniques that are used to render ( a.k.a. approximate :) ) indirect illumination?

• Brute Force
• Irradiance map
• Light Cache
• Metropolis light transport ( is the the same as Light cache?)
• Photon map
• Path tracing, bidirectional path tracing
• VPL ( Virtual Point Lights ) (based on the instant radiosity formulation)
  
• VSL ( Virtual Spherical Lights )

What are the sub-challenges?

• Losing out on small, sharp shadows
  
• Losing out on glossy inter-reflections
• noise!
  

References:
Approaches to indirect illumination used in GI
http://www.spot3d.com/vray/help/150SP1/render_params_gi.htm
http://www.spot3d.com/vray/help/150SP1/examples_GI.htm