VirtualBox is a multi-platform virtualization software that supports Linux, Mac OSX, Windows XP/Vista/7 and Solaris as Host Operating Systems. Originally developed by innotek and later bought by Sun, VirtualBox is a free virtualization solution that can run various Guest Operating Systems like BSD, Linux, OS/2, Windows, Solaris and a few others.
Although not as well-known as VMWare, it offers pretty much the same functionality for free. The new versions support enhanced support for 3D acceleration in guest operating systems, symmetric multiprocessing (SMP) functionality in guests, as well as upto 16GB of RAM per virtual machine. Another great aspect of VirtualBox is that it is open-source (although certain parts of the code aren't) thus you can expect the addition of features and improved functionality if Sun ever decides to stop supporting this virtualization platform.
You can try out VirtualBox by downloading it here (download it for the OS you are running right now, not the OS you want to run inside the VM.) After installation, a wizard will be opened which will guide you through the steps of creating a virtual machine for your chosen guest OS.
After installation/setup of the guest OS, it is recommended to install the bundled Guest Additions inside the guest OS. These will give you enhanced functionality such as higher-resolution display, video acceleration, folder sharing and seamless windows.