Rofi is a window switcher, application launcher and dmenu replacement.

Rofi, like dmenu, will provide the user with a textual list of options
where one or more can be selected. This can either be running an
application, selecting a window, or options provided by an external
script.

Rofi started as a clone of simpleswitcher, written by Sean Pringle -
a popup window switcher roughly based on superswitcher. Simpleswitcher
laid the foundations, and therefore Sean Pringle deserves most of the
credit for this tool. Rofi (renamed, as it lost the simple property)
has been extended with extra features, like an application launcher
and ssh-launcher, and can act as a drop-in dmenu replacement, making
it a very versatile tool.

Note:
If you encounter this this non-fatal error when executing Rofi:
xkbcommon: ERROR: couldn't find a Compose file for locale <your locale>

You may need to open /etc/profile.d/lang.sh and uncomment UTF-8:
# en_US is the Slackware default locale:
export LANG=nl_NL.UTF-8