Windows
Windows comes preinstalled with the Dvorak keyboard layout by going to Control Panel->Regional and Language Options->Languages->Details....
Windows 8 is the 1st version of Windows to support switching the keyboard layout for all open Windows. If you are attempting to switch the layout of all open windows in less then Windows 8 I advise downloading an Autohotkey script. The scripts don't need Admin rights but needs the same rights used as the other windows to work properly. Alternatively PortableKeyboardLayout I believe can work.
I use Dvorak. It is an Autohotkey script that you can that allows yout to enable and disable qwerty and Dvorak between all applications by hitting the CapsLock Key. The scripts don't require administrative rights.
Alternatively you can use a program like Keyla. Maybe even SharpKeys
Portable Keyboard Layout to create custom keyboard layouts in windows.
OS X
Dvorak is one of the preinstalled input sources that can be selected in System Preferences > Language & Text > Input Sources.
Linux
In most Unix based OS's there are two ways in which you can temporarily change the keyboard layout. You can change the keyboard layout in the GUI(x11) interface or in the console interface.
Online
You can type Dvorak online at branah.com/dvorak or inputking.com.
There also have been attempts to detect the keyboard layout in the browser, so you could potentially switch the keyboard layout through the browser online.
Physical Switch
Additionally you can get a physical doggle called QIDO, that switches between Dvorak and QWERTY manually. There are actually keyboard layouts on Amazon.