A fantastic little 2D rocketry and engineering sandbox that will provide countless hours of entertainment – clearly made by a passionate and talented developer, and perfect for lower-end PCs.  
The very aptly named SimpleRockets has a very fun and creative premise: assemble more than two-dozen different types of mechanical modules into just about any shape you want, and taking into account an accurate simulation of fuel usage, weight distribution and air resistance, test it out from a take-off point on any planet in a to-scale representation of our solar system. You have the tools to create multi-stage space shuttles, hardy ground rovers, solar-powered gliders, and just about anything you can think of with the wide array of components.
The longevity of this game comes from the sheer amount of ideas you can incorporate into your designs, and from a selection of challenge scenarios which also serve as a tutorial to introduce you to various aspects of the program. 
While not incorporating any direct multiplayer gameplay, one can enter numerical codes found online at the official SimpleRockets website and associated forums to download the vehicular creations of others, and the Steam port brings with it another set of Badges and Trading Cards to add to your collection.
This is the perfect creativity tool for someone looking to expand on the format they most-likely first saw in a much more simpler form as an online Flash game in years past, or wanted an experience similar to the critically-acclaimed ‘Kerbal Space Program’ without the larger pricetag or PC specs.
Clean presentation, countless hours of fun. Highly recommended.