Windows 7 Family Pack returns (while supplies last)
With the Family Pack, you get three Windows 7 Home Premium licenses, which includes Microsoft's home networking software, HomeGroup. You also get Windows Live Essentials, now with the addition of the Windows Live Mesh syncing app. Basically, this is the cheapest way to go if your household has more than one Windows machine. The only catch is that it's a limited run, so you have to get it while supplies last.
For more info, and a list of countries that will get the Family Pack -- some new ones have been added to the list since the last time this deal was available -- check the official WIndows Team blog.