Close Encounters: Hyperspace Fiends
This very nicely illustrated PDF from Fat Goblin Games (also available in POD) takes the conventional demons and devils of Pathfinder but gives them an SF makeover that would make the Event Horizon proud. The concept of the Fiendish Wastes introduces an interesting concept, in which the planar realms of the Abyss and the Nine Hells have somehow collided and bled through in to the Drift, creating a new dimension which has captured both demon and devil, and in turn both twisted them for the worse and forced them to work together to try and escape their prison, so they can get back to their rightful dominions. To this end the fiends seek to build drift-navigable ships and they need a hyperspace engine to escape their fate. It includes some guidelines on using the concept of the Fiendish Wastes as well as fourteen adaptations of demon and devil for Starfinder as well as two ships and some adventure seeds. The artwork is incredibly evocative and will probably make you (like me) want to find further ways to repurpose these bad boys for your own adventures.
If you get one supplement for Starfinder from a 3PP, I suggest you check this one out. Well worth it, and will go far toward realizing your own Event Horizon incident in the Pact Worlds for sure.
Strange Worlds: Dead Planets
Strange Worlds: Desert Planets
When I talk about good utility in PDFs I mean exactly what these PDFs represent. Both Dead Planets and Desert Planets, for example, are great resources to help build worlds for GMs, and they provide plenty of ready-to-use content for a low price.
Dead Planets is a 16 page PDF which provides an overview on typical dead worlds of science fiction, with details on how to survive, gather resources, deal with airless dead worlds, and wrestle with what made those worlds like they are: total war, destruction by AI, extinction events, or the unquiet, worlds ravaged by the undead. In addition to survival, terrain and threat advice the PDF provides stats for four sample monster encounters: the bloodshade (a terrifying CR 20 undead blob), embalmed ones CR 2 denizens of a dead unquiet world), living holograms (Cr 5 relics of the dead civilizations gone) and overseer robots, who somehow survived the civilization that created them (think CR 10 variant on Halo's Guilty Spark).
Desert Planets is laid out in very similar fashion at 16 pages, with an overview on how starfaring explorers could survive a hostile desert world (Arakis and others), from getting food and resources to dealing with dust storms, flesheater storms, mirages, survival equipment for the desert and so forth. It's not the be-all-and-end-all resource for running your Red Planet or Dune inspired desert campaign in Starfinder, but it gives you plenty for your spacefarers who are jumping around the drift looking to explore random weird worlds. Like the other tome, Desert Planets also includes desert stalkers (CR 7 rat-wolf-cat things), CR 1 dust rats, and the CR 20 sand annelid (sense a theme here?)
So yeah, if you totally want your Starfinder crew to go explore not-Arakis, this book will help you out a lot. Both PDFs are cheap (only $1.95) and in my opinion are some of the most fairly priced PDFs for Starfinder in terms of bang for your buck. I am definitely looking forward to future releases in the Strange Worlds line from Fat Goblin Games, and hope they eventually become available in some POD format, perhaps as a compendium.