TurboTax has provided a free application, TaxCaster, to help you estimate your taxes before preparing a full return. This is useful for running what-if scenarios as part of your last minute tax planning. For example, you can see the effect of things like whether to double up on your property taxes or charitable giving in 2010 or recognizing capital gains (losses).
This screenshot shows the unfortunate case of a single filer with $1M of income and no withholding (oops!).