Nuggan. The god of Borogravia. His followers are forbidden from eating chocolate, garlic, ginger or mushrooms. He first appeared in The Last Hero. He communicates with his followers through the Living Book of Nuggan, a religious text that updates itself within the borders of Borogravia in accordance with the will of Nuggan.
     At least, that's what it did until Nuggan died. Discworld's gods die when they have no true believers left. More precisely, as it was put in Small Gods:

Around the Godde there forms a Shelle of prayers and Ceremonies and Buildings and Priestes and Authority, until at Last the Godde Dies. Ande this maye notte be noticed.

In Nuggan's case, it was not noticed. The people still prayed, but it was to the Duchess Annagovia as Nuggan's representative. The Book still updated itself, too, but the updates made less and less sense as the list of Abominations grew to include the cheese, accordion players and the colour blue.
     While he was alive, Nuggan was described as a tetchy, small god with a fussy moustache. His original, genuine divine commandments included the instruction never to eat ginger mushrooms, garlic or chocolate; that last order was a particularly impractical one, since before it was decreed, Borogravia's most famous export was fine chocolates.
     The New Discworld Companion lists Nuggan as "God of Paperclips, Correct Things in the Right Place in Small Desk Stationary Sets and Unnecessary Paperwork".