Environment Variables
Environment variables for windows 2000/XP
Here below a list of useful windows environment variables:

ALLUSERSPROFILE | Returns the location of the All Users Profile.
APPDATA | Returns the location where applications store data by default.
CD | Returns the current directory string.
COMPUTERNAME | Returns the name of the computer.
COMSPEC | Returns the exact path to the command shell executable.
DATE | Returns the current date.
HOMEDRIVE | Returns which local workstation drive letter is connected to the user's home directory.
HOMEPATH | Returns the full path of the user's home directory.
HOMESHARE | Returns the network path to the user's shared home directory.
NUMBER_OF_PROCESSORS | Specifies the number of processors installed on the computer.
OS | Returns the OS name. Windows XP and Windows 2000 display the OS as Windows_NT.
PATH | Specifies the search path for executable files.
PATHEXT | Returns a list of the file extensions that the OS considers to be executable.
PROCESSOR_ARCHITECTURE | Returns the processor's chip architecture. Values: x86, IA64.
PROCESSOR_IDENTIFIER | Returns a description of the processor.
PROCESSOR_LEVEL | Returns the model number of the computer's processor.
PROCESSOR_REVISION | Returns the revision number of the processor.
ProgramFiles | Returns the location of the default install directory for applications.
SYSTEMDRIVE | Returns the drive containing the Windows root directory (i.e., the system root).
SYSTEMROOT | Returns the location of the Windows root directory.
TEMP | Return the default temporary directories.
TMP | Return the default temporary directories.
TIME | Returns the current time.
USERDOMAIN | Returns the name of the domain that contains the user's account.
USERNAME | Returns the name of the user currently logged on.
USERPROFILE | Returns the location of the profile for the current user.
WINDIR | Returns the location of the OS directory

Environment variables for GNU/Linux
To know all the current environment variables in your system, invoke the printenv program.
Here below a list of some of them:

PATH | System specifies the search path for executable files.
USER | Returns the name of the user currently logged on.
HOME | Returns the full path of the user's home directory.
DISPLAY | Returns the name and display number of the X Window server on which programs will appear.
LANGUAGE | Return set language.
TMP | Return the default temporary directories.
BROWSER | Return the default browser.
DESKTOP | Return desktop currently running.