Bankrate.com lists federal tax bracket tables for the current year and previous tax years dating back to 2008, as of 2015. In addition, an online calculator called "What is your tax bracket?" allows taxpayers to plug in basic financial information and deductions to estimate their current tax bracket.
As an additional source, efile.com lists federal tax bracket tables for the current and previous tax years, and provides an historical summary table of federal tax brackets, tax rates, hikes and cuts going back over 20 years. According to the Tax Foundation, the IRS adjusts about 40 tax provisions for inflation every year to avoid causing people to leap into a higher tax bracket. This phenomenon, called "bracket creep," pushes taxpayers on the edge of two brackets into the higher bracket based on inflation, not an actual increase in income. Staying sharp on IRS updates for the current tax year is therefore important for people close to the upper limit of their bracket.