What are some of the most useful yet little known features in the PowerShell language

The $$ command. I often have to do repeated operations on the same file path. For instance check out a file and then open it up in VIM. The $$ feature makes this trivial

PS> tf edit some\really\long\file\path.cpp
PS> gvim $$

It's short and simple but it saves a lot of time.

By far the most powerful feature of PowerShell is its ScriptBlock support. The fact that you can so concisely pass around what are effectively anonymous methods without any type constraints are about as powerful as C++ function pointers and as easy as C# or F# lambdas.

I mean how cool is it that using ScriptBlocks you can implement a "using" statement (which PowerShell doesn't have inherently). Or, pre-v2 you could even implement try-catch-finally.

function Using([Object]$Resource,[ScriptBlock]$Script) {
    try {
        &$Script
    }
    finally {
        if ($Resource -is [IDisposable]) { $Resource.Dispose() }
    }
}

Using ($File = [IO.File]::CreateText("$PWD\blah.txt")) {
   $File.WriteLine(...)
}

How cool is that!

A feature that I find is often overlooked is the ability to pass a file to a switch statement.

Switch will iterate through the lines and match against strings (or regular expressions with the -regex parameter), content of variables, numbers, or the line can be passed into an expression to be evaluated as $true or $false

switch -file 'C:\test.txt' 
{   
  'sometext' {Do-Something}   
  $pwd {Do-SomethingElse}  
  42 {Write-Host "That's the answer."}  
  {Test-Path $_} {Do-AThirdThing}  
  default {'Nothing else matched'} 
}