Patrick's Harn location downloads, ShaneNine's d20 Harn Guide, and the Harn Religion Team's Agrik page were what sold me on gaming in HarnWorld with d20. These sites offer excellent resources easily used in any fantasy RPG setting or rules.
Harn Religion Team:
http://pages.sbcglobal.net/harn-religion-team/agrik/index.html
For example, the tyrannical fire god Agrik, Lord of the Four Horsemen (linked to, above) can easily become Bane, Loviatar, or Hextor in other settings, or any LE type god of war, hate, conquest and pain. The problem with Agrik's D&D counterparts is they are not detailed enough, IMO. Agrik, on the other hand, has over 100 pages of free online information available! Everything from clerical orders, fighting orders, war chants, rituals, duels, torture techniques, economic and cultural impact, etc. More detail than you could ever hope for in your wildest dreams. And the information is almost entirely about his mortal worshippers and their beliefs, parables and institutions, not useless (to me) info on how to fight the god himself in hand to hand combat, LOL. Hell, there's even a hypertext lexicon of Agrikan terminology! Reading up on Agrik WILL make you better at DMing or playing any LE priest, I guarantee.
Other gods and goddesses at the Harn Religion Team site include:
Morgath, the Shadow of Bukrai (CE god of darknesss, undeath and insanity): His info can be used for Tharizdun, Orcus, Myrkul, Nerull, Shar, etc.
Halea, The Golden Temptress (CN goddess of Luck, Lust and Luxury): Her info can be used for Tymora, Llira, Sharess, Loviatar, Waukeen, etc.
Naveh, the Merchant of Death (NE god of murder, thieves and assassins): His info can be used for Bhaal, etc.
Larani, Lady of Paladins (LG/LN goddess of chivalry, honor and war): Her info (at least 50 pages worth!) can be used for Tyr, Helm, Pholtus, Pelor, St. Cuthbert, Red Knight, etc.
Peoni, Maid of the Ripe Harvest (NG goddess of fertility, love, creation, earth, etc.): Her info can be used for Chauntea, Beory, etc.
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Did I mention that HarnWorld is easily used with the Cthulhu Mythos? You will not find a better setting outside historical earth than HarnWorld for running medieval Cthulhu games!!!