Space Opera and its kitchen sink setting in several posts previously, but what I don't think I've ever talked about (much) is the game that laid the groundwork for both, 1977's Space Marines by A. Mark Ratner. To be honest, I've never seen the 1977 edition of Space Marines, which was self-published by Ratner under the FanTac Games label. Sometime after its publication, the game was sold to Fantasy Games Unlimited, which led to a second edition, published in 1980. That's the only edition I've ever seen, so I cannot comment on whatever differences there might be between the two versions, but I welcome details in the comments by anyone who has seen both.
Space Marines is a science fiction miniatures wargame that uses a scale of 25 meters to the inch and twenty-second turns. The rules are quite comprehensive, covering wide range of topics -- unit integrity, suppression fire, bombing from air and orbit, electronic warfare, morale, and so on. However, the rules aren't particularly lengthy, especially when compared to other SF miniatures games with which I'm familiar, such as Striker. The relative shortness of the rules is at least partially a consequence of the fact that some topics are treated only sketchily. Orbital and sub-orbital bombardment and combat, for example, are largely left up to the referee to adjudicate, with only some very basic guidelines provided in the text. That's not to say that Space Marines is a simple game. It is, however, a lot more clearly written and intelligible than the game it spawned, Space Opera, which, despite my fondness for it, is far from a paragon of clarity.
About a third of Space Marines is devoted to background material. It was this material that Ed Simbalist drew on when creating Space Opera's setting. Indeed, I don't think I ever really understood the full scope of Space Opera's setting, until I'd actually seen a copy of Space Marines. Races and governments to which the RPG only alludes are given write-ups in the wargame. Granted, those write-ups mostly focus on military matters, such as organization, tactics, even uniforms, but at least they exist. Without the benefit of Space Marines, I'd never really know who the Mekpurrs were, let alone even more obscure races like the Rauwoofs or the Whistlers. Ultimately, that's the main reason I still find Space Marines interesting. It works very well as a supplement to Space Opera, filling in some blanks that the RPG's author didn't think needed to be filled lest precious page space be taken away from more important topics like ranged combat status modifiers.