Spartan Games has made its Firestorm Armada rules available as a free download.
Stargrunt WWII adapts the science fiction rules for action in the Second World War.
Beer and Pretzels Space Marines offers rules and a number of scenarios.
Anyone remember Omni magazine? I was a charter subscriber back in the 1978. It was a neat mix of science fiction and science fact. It also — as I remember — had some neat artwork. I don’t remember much more, but according to Wikipedia, “in its early run, OMNI published a number of stories that have become genre classics, such as Orson Scott Card’s “Unaccompanied Sonata”, William Gibson’s “Burning Chrome” and “Johnny Mnemonic”, Harlan Ellison’s novella “Mefisto in Onyx”, and George R. R. Martin’s “Sandkings”. The magazine also published original sf/f by William S. Burroughs, Joyce Carol Oates, Jonathan Carroll, T. Coraghessan Boyle, and other mainstream writers. The magazine excerpted Stephen King’s novel Firestarter, and featured a short story, “The End of the Whole Mess”. OMNI also brought the works of numerous painters to the attention of a large audience, such as H. R. Giger, De Es Schwertberger and Rallé. In the early 1980s, popular fiction stories from OMNI were reprinted in “The Best of OMNI Science Fiction” series and featured art by space artists like Robert McCall.”
The magazine now is available for free online at the Internet Archives.
Here’s an impressive set of Flickr photos with atmospheric Warhammer 40K images.