There was an interesting piece on the radio a while ago about some people who had a religious objection to war - specifically, their religion said that they could not pay taxes if that money would go to funding a war.
That got me wondering - what would be the consequences of trying to accommodate these people? Suppose that your tax forms had a series of boxes which you could check to tell the government what you want your tax money to go towards. Then the government could try to ensure that the war was funded only by the tax money from people who had ticked the "war" box on the tax form, and fund everything else with what is left over.
Of course, the government couldn't guarantee that peoples' money went where they wanted it to go (if they could, many people would probably just leave all the boxes unticked, or just tick the cheapest boxes in an attempt to pay no tax or less tax.
But even as a non-binding thing, the aggregate data could be useful as a gauge of public opinion - if the war is costing much more money than the taxes paid by the people who actually think it's a good idea, maybe it's time to elect a government that will end the war. Similarly for roads, schools, healthcare and welfare. Such a thing might encourage governments to be more open about what your money is being spent on and why.