Spin Out is a set of rules for running Star Wars style pod races. The old micro machines from Galoob work perfectly for figures.
Outland Games presents “Fast Play Mech Rules.” As with a roleplaying game, you begin by rolling up a mech. The entire set of rules occupies just one page.
ToyMallet 40 cents is both a set of fairly well-done generic science fiction rules and a brilliant parody of the Warhammer 40K franchise. Written by Jennifer Reitz, it allows you to use any toys you happen to have, instead of spending many hundreds on “official figures.” The photos of the Hello Kittau Neko-Wai Meowarriors are a scream.
Terragenesis has a nice article with ideas for modeling alien plants.
Brian Gill offers Bugs!, a set of miniatures rules for staging battles with large numbers of plastic insects.