VMware Player is the free version of VMware. VMware is like running a PC on your PC. VMware Player (the free version of VMware) runs any virtual machine created by VMware Workstation, GSX Server or ESX Server. VMware Player also supports Microsoft virtual machines and Symantec LiveState Recovery disk formats. Free is good ;)
Hint: you can download lots of VMware pre-configured OSes from BitTorrent clients.