Poll URL to see if it is up.

PollWebsite.ps1

function Poll-Website([string]$uri){
  $status = ""
	$retryCount = 0;
	while($status -ne "OK" -and $retryCount -le 5){
		try{ 
		  	$status = (Invoke-WebRequest -Uri $uri).StatusDescription
			Write-Host $status
		}  
		catch 
		{ 
		  	<# If the request generated an exception (i.e.: 500 server error or 404 not found), we can pull the status code from the Exception.Response property #> 
		  	$status = $_.Exception.Response.StatusDescription   
			Write-Host $status
			$retryCount++
			sleep -Seconds 5
		}  		
	}
}

Invoke-webrequest is ps 3.0, use older techniques for pre PS 3:

$web = New-Object System.Net.WebClient
$response = $web.DownloadString($uri) #throws on error code