On death, your estate is subject to Inheritance Tax, on sums over and above the nil rate band. Which for the 09/10 tax year affects sums over £325,000. This band can often be exceeded when the estate includes a property.
Where the survivor of a married couple (or civil partnership) passes away, leaving their nil band rate unused, often the case when they left everything to their spouse or civil partner, meaning the spouse exemption applied, this nil rate band can be carried forward and added to that of the survior.
We are more than happy to discuss any IHT issues when undertaking a will review.