$ProcessorCount (Built-in Mathematica Symbol)

$ProcessorCount gives the number of processor cores available on the computer system on which Mathematica is being run.

$ProcessorType (Built-in Mathematica Symbol)

$ProcessorType is a string giving the architecture of the processor on which Mathematica is being run.

$RecursionLimit (Built-in Mathematica Symbol)

$RecursionLimit gives the current limit on the number of levels of recursion that Mathematica can use.

$ReleaseNumber (Built-in Mathematica Symbol)

$ReleaseNumber is an integer which gives the current Mathematica kernel release number, and increases in successive releases.

$RootDirectory (Built-in Mathematica Symbol)

$RootDirectory gives the root directory of your file system.

$ScheduledTask (Built-in Mathematica Symbol)

$ScheduledTask returns the current ScheduledTaskObject.

$ScriptCommandLine (Built-in Mathematica Symbol)

$ScriptCommandLine is a list of strings giving the elements of the command line with which the standalone Mathematica script was invoked.

$SessionID (Built-in Mathematica Symbol)

$SessionID is a number set up to be unique to a particular Mathematica session.

$SharedFunctions (Built-in Mathematica Symbol)

$SharedFunctions is the list of functions currently being shared among parallel kernels.

$SharedVariables (Built-in Mathematica Symbol)

$SharedVariables is the list of variables currently being shared among parallel kernels.