Speaking of clerics of forces/philosophies, here are some examples from various settings:
Planescape
The Great Unknown - Some of the Athar, a faction who expressly disbelieves in the divinity of beings like Zeus or Silvanus (those are certainly powerful beings, but they aren't divine according to the Athar, and they certainly do not deserve any worship), believes that there is something worth worshiping, even if that thing is not personified.
Dark Sun
Elemental Clerics gain their abilities from the elemental planes, not from elemental gods. Gods do not exist as such in Dark Sun, though some clerics personify their elemental patrons.
Jakandor
Storm Priests venerate and placate the impersonal forces of the storms and winds who once brought the Knorrmen to Jakandor.
On the Charonti part of the island, you have Philosophers, who are devoted to the ideal of The Just Society (in various interpretations), as well as the more specialized Jurists who handle affairs of Law. You also have Pantheists who believe there is some truth to all religions, and can gain some magical power that way.
Eberron - this is the motherload.
The Blood of Vol is a philosophy that claims that there is a divine spark within everyone, and that the gods cursed mortals with, well, mortality in order to prevent them from having the time to develop that spark into godhood. They often consort with undead - mindless undead are seen simply as tools, and higher undead are simultaneously revered and pitied. The way the Seekers see it, higher undead have given up their chance at godhood in order to become more powerful servants of the cult.
The Silver Flame is a spiritual force consisting of the souls of almost all the couatl who once resided in the world, dedicated to binding and defeating evil. This refers both to supernatural evil (the Silver Flame is the force keeping most of the Rakshasa Overlords contained in Khyber, deep beneath the world) and the evil in men's hearts (which should generally not be fought with force of arms, but with persuasion unless necessary).
The Path of Light is a philosophy whose followers believe that acting in accordance with it, and spending time in contemplation and meditation, can speed up the cycle of the rebirth of Dal Quor, the plane of Dreams, and change it from the place of nightmare it currently is into something better. It is opposed by the Dreaming Dark, residents of Dal Quor who like the plane as it is, and fear being "recycled" into other beings should the plane be reborn.
The Path of Inspiration is philosophy acting as a front created on Eberron by the Quori. The main tenets are devotion to the Riedran state and its Inspired rulers - think North Korea. Clerics are rare but do exist, and are explicitly not powered by the actual Inspired rulers but by their faith in what they represent. Thus, the Inspired can't remove divine casting from their priests, so if one of them starts getting ideas their main recourse is eliminating the priest.
Homebrew
I'm having some vague ideas about a homebrew where you do have gods worshiped (I'm thinking about ripping off the Dawn War pantheon), but that worship is unrelated to clerical magic. Instead, the various domains represent inherent aspects of reality, and some people have the skill and power to tap into those aspects for various miraculous effects. Some of those would associate themselves with a particular deity, but others would not, and the main function of religion would be social rather than magical. The main benefit of this would be to enable cleric PCs and NPCs without having them tied to a particular church - I always found it annoying the way the Zhentarim were made into direct representatives of Bane in the Realms, and would like to avoid such nonsense.
