Many people have heard of virtual machines and some have even used them in various capacities. I've used VMWare virtual machines for years quite happily, but have had some recent experiences that have changed my mind (a bit).
I use VMs on my TabletPC so I can run Linux, which I'm more comfortable with for software development than Windows. I also run VMs on my MacBook Pro so I can run Windows for various reasons, including Hiperwall software development. When I got the MacBook Pro, I bought VMWare Fusion, because I figured it would be the best. It is very good, but didn't support OpenGL, which hurt the Hiperwall software development, so I didn't use it for a while. Then I saw that Parallels Desktop 4 supports OpenGL, so I bought it and it is great. Things seem more responsive than VMWare Fusion, and more importantly, it supports both DirectX and OpenGL. The only problem I had was that the installation wizard tried to be too smart and handle everything, but got hung up on some dialog boxes that it didn't expect. I sorted it out manually, though, so all it well. Parallel Desktop 4 works very well for me.
On my PCs, I've been using VMWare Server for years, because it is free and fast (once started). Starting it takes forever, though. Really. On my Core2Quad machine at home, it litterally stops the clock update on the display and my Logitech G15 keyboard for several minutes as the VM initializes. On my poor TabletPC with slower disk and (until recently) less memory, it was nearly interminable and intolerable. And, of course, VMWare Server didn't support much in the way of display devices and such, because it was for servers. So recently, I tried Sun's VirtualBox. I tried it a year or so ago and it was terrible, but it has now become really great. It starts right away, is very responsive, and supports OpenGL, so I can run several virtual Hiperwall tiles on one PC for testing! It's just great.
That doesn't mean you should throw away VMWare Server, though, because it does one thing that the others don't. It can let the guest OS use two processors. This is important for multithreaded apps and when I demonstrate parallel programming in my classes. So it still has its place, but not as my everyday VM.
These VMs execute native x86 code directly on the processor, but all I/O (disk reads and writes, network packets, even sound and graphics) is on virtual devices. These virtual devices then use your machine's physical devices to perform the real work requested. A problem is that interrupt handling is expensive for VMs, so most VMs batch interrupts and process them in groups. This means that responsiveness is often not as good as it should be, even though average throughput is reasonable. So playing a game in a VM is probably doable, but it may suffer from stutters and delays due to the nature of the VM's I/O handling.
So VMs are tremendously useful, and though VMWare makes terrific VMs and has been around for a long time, look at some of the others, too. They may meet your needs even better.
