What's the best way to determine the location of the current PowerShell script?

Whenever I need to reference a common module or script, I like to use paths relative to the current script file. That way, my script can always find other scripts in the library.

So, what is the best, standard way of determining the directory of the current script? Currently, I'm doing:

$MyDir = [System.IO.Path]::GetDirectoryName($myInvocation.MyCommand.Definition)

I know in modules (.psm1) you can use $PSScriptRoot to get this information, but that doesn't get set in regular scripts (i.e. .ps1 files).

What's the canonical way to get the current PowerShell script file's location?

PowerShell 3+

# This is an automatic variable set to the current file's/module's directory
$PSScriptRoot

PowerShell 2

Prior to PowerShell 3, there was not a better way than querying the MyInvocation.MyCommand.Definition property for general scripts. I had the following line at the top of essentially every PowerShell script I had:

$scriptPath = split-path -parent $MyInvocation.MyCommand.Definition

If you are creating a V2 Module, you can use an automatic variable called $PSScriptRoot.

From PS > Help automatic_variable

$PSScriptRoot
       Contains the directory from which the script module is being executed.
       This variable allows scripts to use the module path to access other
       resources.

For PowerShell 3.0

$PSCommandPath
    Contains the full path and file name of the script that is being run. 
    This variable is valid in all scripts.

The function is then:

function Get-ScriptDirectory {
    Split-Path -Parent $PSCommandPath
}

For PowerShell 3+

function Get-ScriptDirectory {
    if ($psise) {
        Split-Path $psise.CurrentFile.FullPath
    }
    else {
        $global:PSScriptRoot
    }
}

I've placed this function in my profile. It works in ISE using F8/Run Selection too.