This Flickr album has a series of covers of British pulp science fiction magazines. There’s lots here for inspiration.
1945 WWII Era Skirmish Rules
Tom Hinshelwood offers a set of free wargames rules called 1945, for games set during WWII.
Other Resources:
Star Wars Full Thrust Rules
Full Thrust is an excellent set of starship combat rules — my personal favorite, in fact. Here are a set of modifications and statistics for playing Star Wars battles.
Thumbs Up, Thumbs Down Gladiator Rules
Thumbs Up, Thumbs Down is a set of free wargames rules for gladiatorial combat. The rules are designed for up to ten players.