A couple of years ago I came across Bridgebuilderby Chronic Logic. It was created in 2000 and gives you the opportunity to create 2D bridges to span a given gap, and then run a train across it to see if it is structurally sound. It starts off easy and then the levels increase in difficulty to become fiendishly tricky. The creator of Bridgebuilder has since left Chronic Logic and you can get an updated version from his new site. The total size of the game is only 239Kb so it is quick and easy to download and setup.
I have used Bridgebuilder with a senior science class to look at simple engineering principles such as trusses, arches and the importance of triangles. The physics engine in the game allows you to examine where the stress is distributed in your bridge and is great for illustrating the effects of compression and tension forces.
I have also used it with a year 4/5 computer club as a problem solving exercise. It would be a great game for early finishers in a primary classroom.
Chronic Logic have since released sequels, Pontifex, Bridge Construction set (A.K.A Pontifex II) and Bridge it (essentially a remake of Bridge Construction set with glitzier graphics). All three are available to buy from their site, or you can download a demo. They will do educational pricing if you email them with your needs. The gameplay for these later versions is similar but the bridges are three dimensional and there are new materials to play with including suspension cables and hydraulics for the creation of draw-bridges so that you can make a greater variety of designs. They are brilliant and offer a more involved challenge but it is hard to beat free.
The second game that I feel compelled to recommend is Fantastic Contraption which is similarly themed in that there are levels of increasing difficulty, it is a physics based game, and triangles are king when it comes to stability.
The aim of the game is to move a box or ball from a starting area to a goal area. You have at your disposal two types of rod, self-powered wheels and unpowered wheels with which to move the box or ball. You can fling it with a catapult, build it into a car, send it on a conveyor ride, or pick it up with a crane. There are usually many ways to succeed in the task, and many more ways to fail.
The principles of gravity, pulleys, levers, force, friction and Newton's Laws can all be explored.
Again this is an excellent problem-solving exercise for all age groups. It is browser based so no need to install anything. For an extra US$10 you get access to a gazillion levels designed by the community. You can rate them for difficulty and quality. There is a companion site made by fans that helps to keep track of all of the extra levels. It has been bought by the Fantastic Contraption owners so should become integrated with the main site soon. It also has a wiki that has a page about the physics of the game so you could have your students explore the gravity constant in the FC 'universe'.
I spent waaaaay too much time playing this in during my recent holidays, give it a go.