If you are a local resident, simply use the tax calculator 2009 published previously. It takes into account the 20% special tax rebate.
As the rebate is one-off, for foreigners who would like to get a sense of the tax rates in Singapore, without distortion from the one-off rebate, please use the following salary tax calculator. Note that you’re considered a tax resident if you have stayed in Singapore for at least 183 days. In this case, you’ll be taxed at resident rate.
The non-resident rate is the higher of 15% or the applicable resident rate if you had been a resident. In other words, it’s better to be a resident.
Note further that locals enjoy a host of tax reliefs that may not be available to foreigners.
For example, I know of a lady whose assessable income is $90k but enjoys a total tax relief of $60k! Half of the relief comes from the WMCR. Her tax payable comes up to a mere $300, which is then fully offset by the unused portion of the parenthood tax rebate granted to her 3 years ago.