I find it very interesting that people who consume alcohol get less shy.
Therefore my question:
Why does drinking alcohol reduce shyness?
And are there any other (legal or illegal) substances who have such an effect on the human brain?
It has to do with the excitatory affect alcohol has on almost all neurotransmitters and their receptors, including serotonin, dopamine, and GABA.
Very, very broadly:
GHB will do this even more - but it may do it to way too much of an extent. (Mostly Gaba, but increases tyrosine hydroxylase as well which = more dopamine.)