There are several credits against the tentative tax, the most important of which is a "unified credit" which can be thought of as providing for an "exemption equivalent" or exempted value with respect to the sum of the taxable estate and the taxable gifts during lifetime.
For a person dying during 2006, 2007, or 2008, the "applicable exclusion amount" is $2,000,000, so if the sum of the taxable estate plus the "adjusted taxable gifts" made during lifetime equals $2,000,000 or less, there is no federal estate tax to pay. According to the Economic Growth and Tax Relief Reconciliation Act of 2001, the applicable exclusion will increase to $3,500,000 in 2009, the estate tax is repealed in 2010, but then the act "sunsets" in 2011 and the estate tax reappears with an applicable exclusion amount of only $1,000,000 (unless Congress acts before then).
The estate tax credit or exemption equivalent should not be confused with the federal gift tax credit or exemption equivalent. The gift tax exemption is frozen at $1,000,000 and does not increase, as does the estate tax exemption.
If the estate includes property that was inherited from someone else within the preceding 10 years, and there was estate tax paid on that property, there may also be a credit for property previously taxed.
Before 2005, there was also a credit for non-federal estate taxes, but that credit was phased out by the Economic Growth and Tax Relief Reconciliation Act of 2001.