I found it:
$env:UserName
There is also:
$env:UserDomain
$env:ComputerName
[System.Security.Principal.WindowsIdentity]::GetCurrent().Name
by James Palmer
I found it:
$env:UserName
There is also:
$env:UserDomain
$env:ComputerName
[System.Security.Principal.WindowsIdentity]::GetCurrent().Name