Computer games can be very potent tools to get leftist messages across. For example, strategy games (like Sid Meier's) appeal to those who want empowerment in political/economic systems, rather than just being a cog in the system.
Some political messages are explicit. For example, you're the decisionmaker for a colony, and in certain ways you have antagonism with your citizens. So in "Sid Meier's Alpha Centauri," I recall that converting to a free market economy requires a stronger police force and has severe environmental impacts. It's also sometimes tempting to "nerve staple" inhabitants of a colony to keep them from rioting.
I think capitalism in particular is a good space to design a game, since it's about the economics of competition rather than cooperation, leading to a classical game design.
It would be interesting to put the player in a role they're probably not used to -- in control of a corporation. Antagonism would include not just other players, but the population you're selling to and employing. And unlike in normal life, your role in society is then to make rational choices and ensure your information is good. (As opposed to accepting the media industry's product and passively give others control of your life.)
Incidentally, I think Sudbury Valley school had a healthy viewpoint on games:
"I think fishing was the 1970's version of video games today because people still feel the same way about video games. They watch kids who come to school here and spend tremendous amounts of time playing video games and they see it as mind- numbing. We know – because we see the kids and we know the kids – that it's the opposite of mind-numbing, that they only play a particular video game until they've mastered it and then they go on to something else. They also play in an extremely social situation with other kids, talking all the time, and they develop very deep social relationships. And we know that the kind of concentration they put into each video game is mind-building and not mind-numbing, but people still feel the same way about video games that they felt about fishing in the '70's. It's just sort of a more modern version of it." http://www.sudval.org/01_abou_09.html
Tayssir