The federal estate tax (or "death tax") is imposed on the transfer of the taxable estate of the deceased. Currently, it has a $5.12 million exemption and the tax rate is 35%. The exemption is scheduled to decrease to $1 million and the tax is scheduled to increase to 55% on 1/1/2013 (baring any changes to the current law).
Some states also have estate or inheritance taxes, but can have different rules and a different exemption than the federal exemption (check the specific state you are concerned about).
Estate taxes are based on the value of the assets of the deceased. Inheritance taxes are based on the value of the property a beneficiary inherits. Inheritance taxes can vary state to state and even between beneficiary classes within a state. For example, in Pennsylvania, the inheritance tax is 6% to direct descendants (children, grandchildren, etc.) and 15% to collateral beneficiaries (brothers, sisters, nieces, nephews, etc.). The tax rate is 0% for assets passed on to a spouse.
I believe California has phased out their inheritance tax and Nevada does not currently have an estate tax. Currently New Hampshire and Florida has no estate tax but Tennessee does.
If you are trying to decide what state to retire in, don't just check out income tax rates without checking out estate (inheritance) tax rates. Nevada, NH, Florida, and Tenneessee have no income tax (TN does tax some investment income), but TN does have an inheritance tax. California has no inheritance tax but a horrible income tax.
Remember, you planning has to be flexible. All it takes is an act of Congress or a state legislature to change the rules. What would seem to be a monumental task can occur quickly and almost effortlessly when a state or the federal government needs money.