The estate tax is a tax levied on the estates of individuals upon their deaths. In the early 1900's, when progressivism was strong and politicians became enamored of socialism, the estate tax was heralded as an equalizer. President Theodore Roosevelt endorsed an inheritance tax whose "primary objective should be to put a constantly increasing burden on the inheritance of those swollen fortunes, which it is certainly of no benefit to this country to perpetuate." Roosevelt would later say that the tax should be directed at "malefactors of great wealth, the wealthy criminal class."
"The Revenue Act of 1916 brought back the estate tax and introduced a new tax, the modern-day income tax. Unlike its predecessors, this estate tax would be different. First, it was no longer just about revenue – now it was seen as a tool to address societal inequities. It had become a political tax – a method to accomplish wealth redistribution. As a result, when this war ended, this estate tax did not. Despite sizable budget surpluses, Congress increased rates and added the gift tax on lifetime transfers. The estate tax was seen as a key method to redistribute wealth." 
Just like corporate taxes and regulations, only the largest can afford the specialist accountants and lawyers to defend them. Meanwhile, the middle class gets routinely decimated.
Estate tax charts 
USA Today article 2010-07-21 
Wikipedia article on "Inheritance tax" 
- http://www.fff.org/comment/com1012d.asp A Libertarian View of the Estate Tax, commentary by Laurence M. Vance, December 6, 2010