The Little Black Books

GDW Traveller
Released in 1977 by Game Designers' Workshop, Traveller gave the new hobby a chance to explore the science fiction side of adventure gaming. Over the decades Traveller has been published a number of times by various publishers and has seen several editions, but remains a popular system. With the exception of the GURPS version of Traveller, the core 2d6 mechanic is consistent across editions, a fact which makes even the earliest play aids still relevant. Traveller is currently enjoying a rebirth through Mongoose Publishing's second edition.
The first and second editions of Traveller come as three little black books, perhaps as a nod to the World's First Published RPG. Book 1 Characters and Combat (note the absence of an ampersand) covers just that. This is the famous character generation system that can result in your character dying prior to the first adventure. This may sound a bit unusual, but it makes sense.
Chargen in Traveller differs from most systems. Traveller introduces the career or life-path concept (although they don't use the term) which later re-appears in games such as The Burning Wheel. Once basic characteristics are determined by six 2d6 dice rolls, one each for Strength, Dexterity, Endurance, Intelligence, Education and Social Standing, the player decides which of six careers to try for, Navy, Marines, Army, Scouts, Merchants or Other. If the enlistment roll is not successful, the character is submitted for the draft and a random assignment to one of the six career paths.
Enlistment is for four years during which the character ages and acquires skills and expertise according to random dice rolls. There is also a survival roll, because life is dangerous in the Traveller universe. Failure on the survival check results in character death.
Traveller allows the player to choose re-enlistment or transfer to another career at the end of each four year term of service. It is not uncommon for beginning characters to have 3, 4 or even more 4 year terms under their belt prior to retirement and commencing play. The risk of survival and adverse effects of aging helps balance the accumulation of skills and wealth which can accrue during a lengthy pre-play career.
Traveller is a skill based system, the first I came across in gaming. Skills and expertises are acquired through one's terms of service and through adventuring. A skill or expertise adds to the roll for success during play. Success in Traveller is based on rolling 2d6s, adding the scores and any plus or minus die roll modifiers. The referee may award a plus of minus to adjust for easy or difficult tasks. A total score of 8 or better is needed for success.
I like the bell curve nature of rolling two dice. A frequent complaint of systems based on the roll of a d20 or less often percentiles, is the "swingyness" that can accompany rolling on a flat distribution die. The complaint about bell curve dice is that the plus and minus adjustments mean more or less depending on where along the curve you are. It generally boils down to personal preference.
The simplicity of the Traveller chargen and combat systems lend themselves easily to tinkering. I like games that I can add to and adjust to suit my tastes and preference. Traveller can be used to explore almost any setting - it's a big universe.