James Smith's entry to the Stockholm Green Hackathon was a Minecraft mod that adds carbon emissions to the game, which revolves around resource extraction and use:
When you burn some wood in a furnace, the mod calls out to AMEEconnect to do a calculation, and adds the result to a tracker in-game. As the carbon ticks up, the environment gets more and more polluted as the skies go dark and the clouds come down. OK, not entirely accurate, but an effective visual indicator!
Of course, it's not just wood. Loads of things burn, and not just in furnaces. The hack supports combustion of almost anything in minecraft; wood, planks, coal, tree saplings, and so on. I even put in some calculations for setting fire to cows (as any Minecraft player knows, an effective way to quickly get cooked beef). Even the hostile mobs like creepers have their emissions mapped (mostly to generic biomass calculations). I also added redstone (like electricity) emissions using AMEE's realtime UK national grid data.
Of course, there are also ways to remove carbon from the atmosphere. Plant a tree, and AMEEconnect will work out how much carbon was taken up by the tree growing and reduce the tracker by that amount. After a long day of mining and smelting, you'll have to go plant a few trees to keep the weather nice.