One of my favourite D&D tropes is the idea that if an individual being gains a group of worshipers, it can attain divinity. This makes religion a very chaotic, ad hoc, and above all tangible thing; unlike in the monotheistic religions of our world where Gods stand aloof, D&D powers' fate is inextricably bound up in the fortunes and vagaries of their followers. It also means the interplay between believer and deity is balanced, with neither having the ultimate power; okay, so High Blierophlat the God of Death can squish any follower he so chooses, but if he does that too often he'll eventually find his disciples running off to some more amiable deity like Jethro the Gardener. Unless of course he's willing to offer some serious compensating benefits.
This approach allows for a much more localised, cultish and fertile religious climate, something akin to how I imagine a trawl through pagan Europe would have been: each group of villages has its own local deity, each cave its shrine to the mountain spirit, each river its sprite, and each boulder its guardian - the major difference being that in a D&D world, those things are real, which is infinitely better. The local village deity really will provide summer rains - if the populace are willing to sacrifice a first born. The mountain spirit will kill travellers with an avalanche unless he is properly placated. The river sprite needs just a few more believers before she can attain demi-goddess hood. The boulder is really a galeb duhr who has had divinity thrust upon him by unwanted worship; all he wishes is that his devoted disciples would just leave him be.
A long while ago I wrote a little entry about a crayfish demigod living in a lake, which I think encompasses what I mean quite nicely. I'll be doing a little series of similar articles this week, in celebration of this month's blogger carnival.