Virtual development box
There are a number of virtual development computer solutions. Currently, I prefer to use the Sun VirtualBox, http://www.virtualbox.org/. You can run Windows and Linux solutions on some virtual solutions for Mac. You can also run Linux and alternate Windows versions on virtual boxes on a PC. You cannot run a Mac on a virtual box or a PC to my knowledge. If you are on a Mac, Parallels is rated as a great solution.
If you are running a server, there are several virtual solutions there also. One of the newest trends is a new breed of virtual servers called a cloud. I call it the Cloud server virtual box 2.0 just to have fun with my developer friends who think cloud is the greatest thing since the iPod.