An implementation of constant sized concurrent job queue in PowerShell by Start-Job cmdlet.

ConstantJobQueueProcessing.ps1

$jobQueueLength = 4
$todoList = "Ada", "BASIC", "C++", "D", "Eiffel", "F#", "Groovy", "Haskell", "IO", "Java", "K", "Lua", "VBA"
$job = {
    param($languageName)

    echo "I like ..."
    Sleep -Milliseconds (Random 2000)
    echo "   ${languageName}!"
}

$emptyJob = Start-Job -ScriptBlock {}
$jobQueue = 1..$jobQueueLength | % { $emptyJob }

$loopInterval = 100
$continuing = $true

for (; $continuing ; Sleep -Milliseconds $loopInterval ) {
    for ($i = 0; $i -lt $jobQueue.Length; $i++) {
        $queuedJob = $jobQueue[$i]
        if ($queuedJob.State -eq "Completed" -or
            $queuedJob.State -eq "Failed") {
            $jobArg, $todoList = $todoList

            if($jobArg -ne $null) {
                $jobQueue[$i] = Start-Job -ScriptBlock $job -ArgumentList $jobArg
            } else {
                Wait-Job -Job $jobQueue
                $continuing = $false
            }
        }
    } 
    Receive-Job -Job $jobQueue
}