There have been a ton of awesome games already recommended, so I'll mention a less popular one with a ton of mods - Star Wars: Empire at War. While vanilla version is servicable enough, an enjoyable, if somewhat easily exploited, timekiller that underwhelms but never does anything wrong, there's just an obscene amount of mods out there, that include overhauls for different time periods in Star Wars canon, as well as other sci-fi series such as Stargate. This, for the most part, stems from the fact that there's two different forms of combat - land and space combat, as well as general overworld resources and unit management. There's also many mods out there to vastly improve the overworld AI.