The IRS has a withholding calculator but you have to pretty familiar with tax terminology.
Turbotax has an estimator that uses plain english. Go to the link below and Click on "Tax Estimator". You can change the tax year to 2008. You will need an estimate of the mortgage interest you will pay this year (don't forget to include any points you paid at closing), plus real estate taxes.
TurboTax® Free Tax Tools - Free Income Tax Calculators, Tax Forms, Tax Tips, Tax Help