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

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
}