The Iron Hand site has a tutorial on building a very nice building-sized laser defense battery for 25mm – 28mm science fiction games, such as Warhammer 40K. These would be great as the focal point of a scenario.
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.
Space Rat is an RPG written by Nathan Russell in just 24 hours. It’s inspired by the cheesy science fiction movies of the 70s. It looks like a lot of fun.
Redjack has an attractive photo gallery of figures from the late, but great Warzone game. I’ve got a ton of those figures myself.
There also are a couple of Warzone scenarios and house rules
Legends of Time and Space is Dark City Games’ rules set for tactical science fiction role playing. It’s based on the well-loved, out-of-print Fantasy Trip system.