How do I add a newline to command output in PowerShell?

Or, just set the output field separator (OFS) to double newlines, and then make sure you get a string when you send it to file:

$OFS = "`r`n`r`n"
"$( gci -path hklm:\software\microsoft\windows\currentversion\uninstall | 
    ForEach-Object -Process { write-output $_.GetValue('DisplayName') } )" | 
 out-file addrem.txt

Beware to use the ` and not the '. On my keyboard (US-English Qwerty layout) it’s located left of the 1.
(Moved here from the comments – Thanks Koen Zomers)