A Tax I Can Support
Believe it or not, but there is a tax that I could support as libertarian. In fact, it's a tax I even would want to see. Because it fixes the problem of underfinanced government budgets and it also aligns the voting populace with the risk of "investing" in government. The incentive problems with government are basically resolved by replacing the current tax code mess with this one tax.
Here's the principle behind it: those who exercise their "right" to vote also pick up the tab for voting. In other words, the tax base consists solely and exclusively of those who voted in the previous election. Voting, in other words, not only provides you with the "right" to choose your government, but also locks you in as a party to it until next election.
Rather than a lump sum tax, which would force poor people and tax consumers out of voting, I envision this tax as a proportional percentage of earned income. So if you vote and the government budget is 75% of the total income of the voting fraction of the population, then the tax rate is 75%. So government cannot spend more money than those in support of it actually have. The budget must balance revenue and expenditure, but this is no problem as long as the population lives beyond its means (that is, when government spends more than 100% of voters' income) - the tax can simply be increased.
We often hear rich people say they don't pay their "share," but this would solve this problem. And it is possible progressives would support this idea too, since those who earn no income can vote without any risk to their own capital. They can actually vote to get their "share" of the rich people's wealth (at least, those who are in on the system). So it redistributes the income of voters in a "fair" (read: you get what you deserve) way.
Of course, the rest of us - us non-voters - would not be subjected to taxation. While there might be blame of "free riding," it is easy for government to simply step out of road-building if it is too expensive. Voters decide by choosing the candidates who promise to spend their (voters', that is) monies on prudent things. If they spend on non-exclusive goods, then that's their and their voters' problem. It is a matter of generosity toward the poor non-voters as well as a cost-benefit analysis: what's really in it for these people to maintain the government highway system? If it is not valuable enough, perhaps government should step out of it altogether (privatize, that is).
So here's a tax that would solve most of the really difficult problems of modern-day democracy. Taxes are used where they're needed, and in order to have a say you have to put your money where your mouth is. Government would consequently be more legitimate, since its services are provided only to those who vote: that is, those who support government enough to pay for it and those who lack an income and want government handouts. The bond between decision-maker and subject... eh... voter becomes stronger too.
It also releases the entrepreneurial powers of the voting populace, since their incentives would be nicely aligned with their true interests - and their pocket books. In other words, they would vote for what they really want and are willing to pay for, not simply what feels or sounds good. And if there's nothing good enough available, there's always the option of not voting and thereby not having to pay for it.
Of course, someone might say that this is not real taxation because people can opt out. And that's true, I'm standing with one foot in Dreamland. What would government be without its ability to coerce all of us whether we want it or not? It would be a different world.