What naming scheme do most professional use?

It seems to me that the underscore is usally saved to be used as part of the prefix 'wp_' (wordpress), 'ipb_' (invision power board). After that I've seen people use diffrent things. Some seperate word like Entity ID with an underscore 'entity_id', other merge them toghter as one word 'entityid', and still other captilize the first letter of the next word 'entityId'.

I realize its completely up to whom ever to use whatever but I still imagine one scheme is more common then the other.