It depends on what you are looking to do... If you are looking for something to occasionally run a vm, or have different OS's running all the time. OpenBox and VMWare are pretty nice tools for the occasional virtualization. ESX or XEN are nice when you plan on having many virtual servers running. ESX can be picky about hardware (namely NIC's... it didn't like the Realtek chip last time I tried). Like I said at the beginning, you will want a tool that suits your needs here. you probably don't need ESX to run an occasional VM (though you could do this) and you probably wouldn't want to run a 24/7 server off of an OpenBox VM (though it could be done as well).