For the most part, it isn't quite as good as the above options, but it does come with a free Windows XP license and good integration with Windows Explorer and XP mode, for running those old programs that Windows 7 doesn't play nicely with. If you're only virtualizing Windows, you also have the choice of using Windows Virtual PC. If you want the whole package, VMWare Workstation has everything VMWare Player has and more (like snapshots), but it'll set you back 200 clams, so it probably isn't worth it for most home users. VMWare is also feels a bit more sluggish, though like VirtualBox, it is free, so it's worth trying both. The main differences are that VMWare's equivalent of seamless mode is a bit better integrated and it has drag-and-drop file sharing, though it doesn't have a snapshot feature-which is, arguably, a more useful feature, which is why VirtualBox ekes it out in this App Directory. VMWare Player is VirtualBox's main competition, providing a similar feature set from a well-known company in virtualization. But, overall, it's still very feature-filled, and with a great documentation and a ton of users, it isn't difficult to find answers to any of your questions. In addition, its "seamless" mode, while cool, isn't done quite as well as VMWare's-it brings the entire toolbar of your guest OS with it, and moving the Windows around isn't the smoothest experience. VirtualBox can seem a little intimidating to most beginners, but so can any virtualization program.
You can even share your clipboard back and forth between your virtualized and host OS. It integrates with your mouse pointer, so you don't even have to click on the window to start using it, and lets you create "snapshots" of your machines so, like restore points, you can just boot it up from any point in its history and use it from that point. Just insert your install disc (or point it to an ISO on your computer), and you can install it in a virtual machine with as much or as little RAM, CPU, and hard drive space as you want.
VirtualBox makes running other operating systems-whether it be Linux, other versions of Windows, or even Mac OS X-super easy on your home computer.