If you are using Windows Vista or better, you can pipe text to clip.exe to copy it to your clipboard:
Dir $env:windir | clip
Here is another approach that you can use if your PowerShell host uses the STA mode:
function Set-Clipboard { param( $text ) if ($Host.Runspace.ApartmentState -eq 'STA') { Add-Type -Assembly PresentationCore [Windows.Clipboard]::SetText($text) } else { Write-Warning ('Run {0} with the -STA parameter to use this function' -f $Host.Name) } } Set-Clipboard "Hello World"