One of the few benefits in the tax system of being married is that there is no IHT on legacies to one's spouse. Doubtless Greedy Gordon will clamp down on that too ...
Actually that one comes with a bit of a health warning too; sometimes that is the worst thing to do!
At present you don't pay Inheritance Tax on the first £275k of an estate. Let's say that my wife and I own a house worth £500k and have £50k in the bank. If she dies and leaves her share to me there is no tax to pay; however, if I then die a week later and leave everything to the kids then they will have a tax bill of 40% on £550k less the £275k exemption, which is a tax bill of £110,000.
If, however, she leaves her half of the house to the kids when she dies and leaves the cash to me then there would still be no tax to pay on her death (the £250k half share in the house is less than the exemption). If I die a week later I am leaving my 50% of the house plus £50k cash to the kids, which totals £300k. Take off the exemption, and the tax bill on my death is 40% of £25k, or £10,000. Your kids are £100k better off, and the taxman gets £100k less.
As with any financial advice, you get what you pay for (so don't take any notice of free advice on here or any other websites, go and spend a bit of money on a decent solicitor or accountant!!), and the most important thing is NEVER give money or property to the kids if you are not 100% sure of your own future financial security (if, say, you fell out with them).
Oh, and one final thing. There is something called a "Deed of Family Variation" which, as long as ALL the beneficiaries to a will agree, means you can retrospectively change the will to make it more tax efficient. So, for example, in the situation above the kids and I can agree and "override" my wife's will after her death so that half the house gets left to them instead of to me. If you ever end up in a situation like this get GOOD advice QUICKLY!