superkey-launch allows you to open KRunner in KDE using the Super key
as well as the app launcher in XFCE and Mate.

If you hold down the Super key it will still act as a modifier key,
allowing you to use it for other keyboard shortcuts. 

superkey-launch is desktop agnostic.

superkey-launch is a small application that runs in the background as a daemon
It was forked from xcape by Albin Olsson: https://github.com/alols/xcape