Alex Schroeder created a rules-light RPG loosely based on the principles of "classic" Traveller, the science fiction game - more specifically, the 1977 or 1981 versions of the so-called "little black books" with some inspired parts from the latest edition of Traveller.
The German version of the game is called Helmbarten, and Peter Frölich and I helped translate that into the English version called Halberts. Four of us - Alex as referee, Peter, PresGas, and I - have been playtesting the included setting and the rules. Notes on our sessions and characters can be found here.
Unlike Paul Elliot's Mercator or Doc Grognard's Adventurer, Helmbarten is not a straight historical or fantasy adaptation of the original Traveller rules, and doesn't require having access to any of the Traveller books. Instead, it draws inspiration from what sets Traveller apart from other RPGs with origins from the same time - simple task resolution, easy character generation, and (perhaps the biggest difference of all) entirely front-loaded character advancement.
It doesn't focus on, or even have rules for, gaining experience or item acquisition to increase character power during play - as a player you determine how much risk you want the character to undergo to continue to advance at the time of creation, and then you play that character.
To explain a bit:
A character in Helmbarten doesn't advance in the fashion of D&D and the other RPGs inspired or derived from it. A Helmbarten character develops in 4-year blocks at the time of creation. At the end of each 4-year stint, the player determines if misfortune strikes the character, and (presuming the character hasn't died) determines if they want to continue in the same career for another 4 years, take a year off to attempt to switch careers, or simply begin adventuring at their current state of advancement. Increasing age and increasing likelihood of a stroke of misfortune are weighed against gaining new skills or increasing those the character already has, as well as the final material or social benefits the character will have once they start adventuring.
The lack of resource acquisition and management during play really makes Helmbarten play differently - you're not out to gain money or experience to improve your character's hit point total or gain the next feat, or learn new spells, and there's no tree of magic items to "climb" to ensure you have the proper power level or build. Your character's survivability (their three physical stats, combined) represent all of the "hit points" your character will ever have.
The default setting is a fantasy Alpine region with a Merovingian feel, but the framework for populating the game world and fleshing it out make it really easy to develop different settings. I myself am working on putting together a few "supplements" that are based around the Sea Peoples of the Mediterranean, an Avestan setting, and an ancient-era Fertile Crescent inspired setting.
As I mentioned earlier, the four of us have been playtesting. I've become quite immersed in the setting, and I have particularly enjoyed the way the process of character creation generates a character and then gets out of the way of gameplay. No messing around with experience points or gold or consumable resource management - you just "run what you brung".
Our game sessions are short by modern conventions (roughly two hours every two weeks), and our "backstory" is comprised mainly of what we generated through random rolls on the career tables as we generated our characters, mixed with emergent backstory developing through our interactions with Alex's NPCs. It plays like it reads on paper - a rules-light game system for a short series of adventures that everyone just has a lot of fun with. For me it's also been really immersive in ways I didn't expect.
Maybe it's related to the way that the character advancement is out of the picture once you've created a character. It could also be that I am playing with great players (Peter and PresGas), or that Alex is a fantastic referee, or that because our sessions are short we really focus on playing. It might be that this game really strikes on the key points of what I've always wanted from an RPG. I suspect it's all of that combined that has made this one of the most enjoyable games I've ever played in.