A drawer.
The person who writes a check is called the payor. Or in the case of my brother, a sucker, when he writes another check for his girlfriend.
The person who makes a will (or 'testament') is a testator.
A Philosopher
A person who writes the libretto for an opera is simply referred to as the librettist.
proposal
publisher
A poetry
A person who writes verses is a poet.
A columnist...?
The word is referee.
Procedural Author
A screenwriter