The Heritage Foundation, Taxmageddon: Massive Tax Increase Coming in 2013, by Curtis S. Dubay:

If President Obama and Congress fail to act this year, an enormous, unprecedented tax increase will fall on American taxpayers starting on January 1, 2013. … This impending tax increase is mostly the result of the expiration of many long-standing policies that all expire at the end of 2012. … Taxmageddon is a $494 billion tax increase that strikes at the beginning of 2013. Under current law, tax policies in seven different categories will expire, and five of the 18 new tax hikes from Obamacare will begin:

Almost 34% of the tax increase from Taxmageddon comes from the expiration of the 2001 and 2003 Bush tax cuts. These cuts are best known for reducing marginal income tax rates, but they also reduced the marriage penalty, increased the Child Tax Credit and the adoption credit, and increased tax breaks for education costs and dependent care costs:

For further details, see here.