Mantic offers a free pdf version of its Deadzone science fiction rules. I played these at a convention in recent months and enjoyed them very much.
science fiction
Laserblade Science Fiction Skirmish Rules
Laserblade is a set of free wargames rules for science fiction skirmishes.
Building A 3D Space Hulk
Although it was essentially a rip-off of Aliens, as a game, Space Hulk was one of GW’s best game efforts. Here’s a tutorial on building your own 3-D Modular Hulks.
Mecha!/Spirit Warrior Rules
For a limited time, Scale Creep Miniatures is making its classic Mecha!/Spirit Warriors rules available. These feature big stompy robots with an Aztec flair.
Free Solo Science Fiction Miniatures Rules
Sally Forth Miniatures offers a free set of rules for playing solo Science Fiction miniatures games. While designed for ACP164, they are adaptable to any sci-fi miniatures system, such as WH40K, Void, Star Wars, etc.