Rep. John Campbell (R., Ca.) has had enough:

There are many liberals who espouse the virtues of big government and say that they would be willing to pay more taxes. I feel they should have the ability to put their money where there mouth is. That is why I am introducing the Put Your Money Where Your Mouth Is Act, which would amend the Tax Code to allow individuals to make voluntary donations to the federal government above and beyond their normal tax liability. The bill would even place a line on the IRS tax form to make it easier for them to do so. Leaders should be willing to do things before they would ask others to do them. Let’s give them a chance to do the honorable thing and pay extra taxes before asking others to do the same.

I think some other countries have such provisions. I am not aware of any substantial take-up of the option.