What are all these metal cans/pods sitting on top of electronics?

Mass loading is a load of tripe.  Pure snake oil.   As others have mentioned; I sometimes place heavy objects (the stainless doorstops work nicely) on small electronics to prevent the cables dragging them around.  

I also do this for my multiport USB chargers.

Add "bee bees"!?  Talk about introducing a "buzz" into an audiophile system....

I use empty, crushed cans of Guinness on my components.

I get dynamic, expansive, 3D memories.

Black light posters, speakers in nets, lava lights...real tweaks. And those light machines that synced with the music. True enhancements. Not a bunch of boring rocks hyped by equally boring mass loading shills. No mass. 

They are a tweak for vibration and resonance control in components.