It's not really that hidden, but may be so for those with no experience, but I really like being able to give lists and let bash expand them like in:

cp arq{,.bak} 

which is the same as typing

cp arq arq.bak 

I also use the history shortcuts (I don't think that's the right term, but...) like

!! 

to repeat the last command, or

^foo^bar 

to replace foo by bar in the last command