Virtual Machines

If you know about virtual machines (that lets you run another operating system on top of one, at the same time), then you probably know about VMware. Their products are mostly paid, but they have a free VMware Player which can run virtual machines created by others. Yet, you cannot even easily modify the virtual machine settings such as the amount of memory.

A competitive free alternative to VMware is Sun VirtualBox. Since version 3 of VirtualBox, their performance benchmarks are similar, it also supports multiple virtual CPUs, shared folders between the guest machine and the host, and various operating systems. It has a very nice seamless mode, similar to VMware Fusion on Mac, that windows from both machines are displayed on the same desktop. With VirtualBox tools, seamless mouse and keyboard integration is perfect. You can create your own virtual machines, or download pre-made images.

Hint: Use fixed-size virtual hard disks. With dynamic ones, the hard disk gets big fast, and then there are painful tricks to shrink the size, and they don't always work. You can always add more virtual hard disks later on.