Thought about this a lot over the years...
I'd love it if we were given the option to choose where our income tax dollars went. For example, log onto the IRS website with your income tax ID number, then pick which departments of government you wish to fund and even allocate your tax dollars as a % to each section, or sub-section, of the budget for the selected fiscal year. E.G., I might put 25% into Defense, 25% into the Veterans Administration, 25% into the Courts, and the last 25% into NASA for fiscal year 2012.
Since each of us have different priorities, and views about the role of government, each person could see their income tax dollars put to work on only those specific areas where they agree with that particular government expenditure while no longer being forced to contribute to government spending in areas they disagree with.
Such a change would radically alter the way government functions. Many of the lesser known and/or unpopular departments and expenses would see their budgets disappear entirely while others would see an increase in their budget. When the Iraq war started it was very popular, and would have seen ample funding, but as it became less popular, the funding would have dried up and potentially saved us billions of dollars by getting us the hell out of there sooner.
Thinking out loud here... I'm not sure anyone would voluntarily fund the IRS, and since they would be necessary for such a program, perhaps something like .25% (leaving you with 99.75% to divvy up) should be automatically set aside for their funding as a "fee" for the service being provided. I'd gladly fork over a fraction of a % of my income for the opportunity to decide where my income tax dollars went..