Hypervisors are applications, or operating systems, or both, that allow you to run one or more virtual machines on a single set of hardware. They come in two flavors: (1) applications that run on a workstation to allow it to run a VM, and (2) "bare metal" hypervisors that act as stand-alone operating systems. For now this page focuses on the former.
Owned by Oracle.
Free download at VirtualBox.org.
Runs on Windows, Mac OS, Linux, and Solaris.
Owned by vmware, the same folks who make the vmware bare metal hypervisor.
Free download at the Download VMware Player page
Runs on Windows and Linux
Commercial Product, about $80.
Mac OS.
Info at the Parallels Desktop for Mac page.
Disclaimer: I own several copies of this product and use it daily; that does not mean that the other options won't work as well or better for you.