From what I vaguely remember from a dig long ago, the 16th amendment which was in mid 1800's gave congress authority to levy taxes on income.

However, the income tax that we pay is really a payment to the federal reserve as interest for the free money they print. At least the part that goes towards the fed is unconstitutional, as it can be argued that the fed itself is unconstitutional since the constitution only allows the states to print money.