Agents in financial markets who exist just to facilitate (and cream off) financial market transactions.
Often a lot of these roles are performed by the same organisations - banks. State-imposed regulations seek to force separation ('chinese walls') between different financial intermediary roles of banks, where it is perceived that overlap between these functions could be anti-competitive or otherwise hinder efficiency of markets.
On the other hand, a contrary tendency is for global banks to become ever bigger, taking on more and more subsidiaries and different functions. But at the same time there is the rise of the 'non-bank banks' separating out particular bank functions. And in general movements (and counter-movements) towards what is called 'DisIntermediation' - as financial markets become more developed and more efficient (?) do needs for intermediaries eventually wither away?
All these themes and more could also be discussed in OnBanks.