Harn (more correctly Hârn) is a fictional island on the world Kethira, created by N. Robin Crossby in 1983 for the game of the same name. The game is currently published by Columbia Games as HarnMaster, along with a tremendous wealth of supplements. In addition, the author has published his own edition, which is slightly different, as HarnMaster Gold. Currently, the latest edition of HarnMaster is the second. There is work being done on a d20 conversion, which is the latest trend in gaming systems. Finally, there are freely available conversions on the web which would allow you to use the Harn background (which is its primary strength) with GURPS, Hero System, or RuneQuest. Since this node is about Harn, I will concentrate less on the roleplaying game, and more on the background. I may fill in more details about the actual system in a HarnMaster node.
Harn is praised for being one of the most "realistic" of the fantasy genre. The world is magic-poor, without being completely without magic. The economy is such that you probably wouldn't find a trove of thousands of coins, let alone tens or hundreds. And the world has been given extensive detail, with an eye of keeping it close to real world medieval Europe (specifically the United Kingdom).
Given that the world is magic-poor, you won't have your typical dungeon delve where you fight beholders, hook horrors, or owlbears. You're more likely to hear about a rampaging monstrousity going through the country side, defeat it after a long battle, and the peasants will tell the tale for generations. You're even more likely to end up in a politcal intrigue in the Thardic Republic to prevent a Senator from becoming too powerful, or prevent an assassination attempt on the king of Rethem. With that said, there are still orcs in Harn, just they're called Gargun.
There are nine kingdoms in Harn (two non-human) and eighteen tribes. The two non-human kingdoms are Evael (an elven, or Sindarin kingdom) and Azadmere (a dwarven or Khuzdul kingdom). There is little contact between the different races, so the game is primarily one of humans.
The amount of detail that you can find on any one aspect of the world is amazing. There were originally 16 books called the Encyclopedia Harnica. Now there are a tremendous amount of other materials available. There's the core rules, HarnMaster Magic, HarnMaster Religion, HarnManor (rules for strongholds), HarnMaster Barbarians, Nasty Brutish and Short (info on the Gargun), Cities of Harn, Castles of Hârn, Castles of Orbaal, Heroth Castle, a ton of maps, and a book for each kingdom and tribe: Azadmere, Kaldor, Chybisa, Orbaal, Tharda, Rethem, Kanday, Melderyn, Araka-Kalai,
Castles of Orbaal, Evael, Ivinia, Menglana Shorkyne.