OGame
As of version 0.78 people can hire a merchant to trade one resource type for another. This is almost the same as trading except that there are no other players involved. The only limitation you have is the amount of free space in your storage silos and the amount of dark matter you have. You cannot trade when your silos are full, or when standing on a moon. It is wise to move away all the resources you want to keep, so that you can trade the maximum amount that your storage holds.