I haven't seen it mentioned in this thread yet, so here's my plug for The Primal Order.
Find it, buy it, swear by it.
The short version:
This book takes a look at gods and offers rules for them that can be applied to any RPG. Basically, the same power that wizards use to cast a fireball, or mentalists use to read minds, or druids use to wildshape, etc., is all just a shadow of Primal magic. To use Primal magic, you need to be a Primal entity. How you become a Primal entity can vary, but typically for a guy to start out mortal (a PC of any usual D&D variety) then become Primal, he needs to receive a Primal infusion from some other Primal entity. This weakens the giver but strengthens the receiver - most Primal entities don't do this lightly.
Once you have Primal energy, you can tap into the raw power of the universe. You don't need spells, you don't need rituals. Your power is yours to bid as you see fit. You get some replenishable reserves (for simplicity, the reserve replenishes daily) and you get some base energy that doesn't replenish - ever. You can spend both kinds of energy, but spending your base becomes a slow road to suicide.
Primal energy is far more powerful than other kinds of energy. The simplest Primal Bolt would blast straight through a Prismatic Wall as if it wasn't even there, and the simplist Primal Shield would stop a Maximized Enlarged Meteor Swarm.
With Primal energy you can build things, destroy things, answer prayers of your faithful, etc. But since your reserves are limited, you need to spend your energy wisely. The most efficient use of Primal is to trickle it into other, simpler activities, such as using a normal human to lift a giant boulder and infusing his muscles with just enough Primal to get the job done - this uses less Primal energy than doing some sort of Primal Telekinesis to lift the same boulder.
You increase your Base and Daily Primal by gaining followers, building temples and shrines, gaining domains (e.g. being the God of Fire), gaining planes (e.g. being the god who rules the Plane of Fire) and you can take Primal from other entities by force, sucking the divine life out of them.
This book gives a very interesting set of rules to explain how to become Primal, how to use Primal energy, and how to exist and grow as a Primal entity.
It's a very fun read, and very easy to incorporate into just about any RPG. I have used it 3 times in 3 long epic campaigns in which players eventually gained some Primal energy, then had to figure out what to do with it, and had to face Primal entities that far transcend the stuff you find in any Monster Manual.
Great fun for all.