Game System: Lamentations of the Flame Princess.
The gist: It's old-school D&D (Basic/Expert base rules) with a twist: It's got weird magic, it's got a ton of metal influences, and it's set up to play in an "early Modern period" instead of the default "pseudo-Medieval" of Dungeons & Dragons. What I like is that it's a fairly straightforward, simple rule set in which you can easily make a character in 10 minutes. And that's great, because all characters are fragile and the world is a bizarre and brutal place.
It is June 1, 1618.
Atlantis has re-appeared in the North Atlantic after a two-hundred and twelve year absence. She is still inhabited by descendants of the brave French and Spanish settlers who risked all to settle her shores between her discovery by Admiral Diego de Camiña on August 4, 1450 and May 14, 1506, when the entire island disappeared without a trace.
The first continental power to re-establish communication (and thereby dominance) over the Atlanteans stands to profit substantially. Atlantis is rich in natural resources, as well as exotic creatures and rare metals found nowhere else in all the world. Whichever crown calls Atlantis her ally (or, better yet, her vassal) will stand tall above her enemies - and in these uncertain times, friends are few, and enemies are everywhere.
As a group, you'll need to choose a patron:
France. You are part of the entourage of Roul Pasquier, a Parisian diplomat. Your duty is to guard Pasquier as he conducts affairs in Soluna, Atlantis' capital, as well as to perform any 'unsavory' duties he assigns to you. Pasquier is a wealthy man, and his family owns several iron mines. He wants you to discover new sources of mineral wealth on Atlantis, and to buy, cajole, bully, coerce, and otherwise take the rights to that wealth. For the glory of France, of course.
Rome. You're part of the Vatican's expedition to Atlantis. Officially, you're there to act as guards for the esteemed Cardinal Francisco Bertello as he conducts affairs in Soluna, Atlantis' capital. Unofficially, you are relic hunters, acquiring magic, art, and treasure for the Catholic church. You are also tasked with endeavoring to understand how an island could disappear and re-appear in such an inexplicable fashion, and to root out and destroy heresy and darkness wherever it might appear.
Independent. You're mercenaries, adventurers, and privateers. Although you're under-funded and don't have any crowns backing you up, you're also completely unfettered by their expectations. Your story begins in Soluna, Atlantis' capital, shortly after you part ways with the Spanish merchant vessel which brought you here from Europe.
And if you're a GM wondering about influences and such:
The Default Setting. Essentially the skeleton I'm hanging the rest of the game on, at least as far as adventures in Atlantis go.
The Inspiration. Turtledove's prose is overly verbose and repetitive, but his ideas can be pretty neat sometimes.