Monotheism
To maintain consistency, I decided to create a system of deities/allegories such as love, war, temptation, despair, virtue, spring, fall, winter, fire, etc. These are the background gods, the "real gods" if you will. For each culture in my world, I then designed a religion that interprets these "real gods". Mechanically, the emotional/humanocentric gods (love and war, for example) are confined to the Ethereal Plane, as they are really just concatenations of like-minded souls (the idea is stolen from Warhammer's Realm of Chaos). These entities can only interact with the real world via their outsiders, who then become the basis for the gods people worship.
The nature allegories (spring and winter, for example) are tied to the real world, and I made them into the kings and queens of the various fey races. They are generally quasi-deities (divine rank 0) and epic level characters, which keeps them more powerful than the PCs but also makes them tangible and within reach - somewhat like the way the gods interacted with people in folklore and mythology, and in the Hercules and Xena television shows.
So, to get on topic, yes, there are monotheistic religions in my campaign, usually worshipping the God of Law ("I am a jealous god, yadda yadda yadda). My system allows me to combine monotheism and polytheism (and animism) in such a way as to create conflict, which is the basis for adventure. The God of Law, for example, isn't the only true god in existance, but he expects his followers to behave like he is, and he expects his clerics to drive the point home to the unenlightenend who would argue otherwise.