A comment to the last post asked where to get started with Traveller. We're talking about a game has had nearly as many confusingly-different editions as D&D, so there will be many different opinions on this matter. I assume (and hope) that fans of other versions will chime in here.
First, let me start by telling you what I use. My preferred ruleset is the 1981 version of Books 1-3, the slight update and reformatting of the original game. In some ways it is comparable to the Original Collector's Edition of OD&D: the differences between the original version and this update are so slight that a casual non-nitpicker will miss many of them. I also like to throw in Supplement 4: Citizens of the Imperium for added PC options. It's not necessary, but most players like to have more classes to choose from than the six in the original game.
Any serious gamer could do just fine with the rulebooks I use, but there are other equally good options. Starter Traveller was designed to be the Basic D&D of Traveller sets and I think it really hit the bulls-eye in terms of providing the most newbie-friendly Trav experience. And it features a spiffy Deitrick cover. Like my Books, this one is long out of print. CAVEAT: When buying a copy make absolutely sure the separate tables & charts sheets are intact. Most of these charts are not repeated in the rules and the game is unplayable without them.
Now, if you want to get into the Classic Traveller experience but don't want to hit the secondary market for musty old tomes from the 80's, you actually have a surprising number of very good options. Marc Miller (the number one Traveller guy, the Gygax of the scene) will sell you either awesome reprint editions or all-in-one CD-ROMs. I have the reprints of all the original Books, Adventures, and Short Adventures in three cool landscape volumes and I totally dig 'em. The Classic Traveller CD-ROM has all that stuff plus the Supplements and Games and probably some stuff I'm forgetting about, all for $35 bucks. That's seriously the best value in the hobby since the Dragon Magazine CD-ROMs from a few years back. There are collectible items on that CD-ROM that will cost you more than 35 bucks by themselves.
But the big reprints and the CD-ROM can definitely cause overload. You ever get that feeling that you received too much setting or system in a game? Traveller with all the bells and whistles can hit you with a double whammy of that sensation, as the advanced systems plus the mammoth 3rd Imperium setting can be a lot to swallow. To slim down the rules to the minimum necessary I recommend the Basic Books 1-3 Reprint available from QLI. That's the rules I use from '81 with some more modern spit and polish applied to the production values. And an alien dog man on the cover. If you want to use the 3rd Imperium setting you can do that basically for free nowadays. Just Google "Traveller Library Data" and start reading. The rabbit hole of online Traveller geekery goes as deep as you want to pursue it.
So I hope that answers the question. There are certainly other options. MegaTraveller is basically the AD&D of Traveller, as it takes a bunch of the advanced options from the original game and incorporates them into the text, along with other complicated subsystems. I'm not a fan personally but it has its adherents. Traveller: The New Era dropped the original mechanics in favor of a port of the Twilight 2000 rules and rewrote the setting as a post-apocalypse in space. It hardly looks like the same game to me. Traveller 4th Edition, a.k.a. Marc Miller's Traveller, tried to turn back the clock on the setting while offering a new resolution system. I never quite got the point of that edition but I like one or two of the supplements. If you like GURPS than I heartily recommend GURPS Traveller. If you don't like GURPS many of the supps are still awesome-tastic.
Finally, just around the corner are at least two new editions, Marc Miller's long anticipated/dreaded T5 and the Mongoose Publishing version soon due out. I stopped following development of T5 a while back when it seemed like it was going to use a resolution system similar to T4. I don't really like the T4 resolution mechanics as I understand them and much more importantly, I think a single resolution system for all actions is not the right way to run Traveller. Detractors of the original rules like to say that Classic Traveller's big flaw was lack of unified resolution mechanics. I consider that a feature, not a bug. I haven't followed Mongoose's version because at this point if I can't follow Marc Miller's lead on Traveller then I don't see much point on following anybody else. I've got the books I like and am happy to make my own way with them.
4 hours ago