There are quite a few of these Quicksilver-clones these days:

* Ulauncher http://ulauncher.io/

* Albert https://github.com/albertlauncher/albert#albert-

* Synapse https://launchpad.net/synapse-project

* Kupfer https://kupferlauncher.github.io/

* Zazu http://zazuapp.org/

* M-x counsel-linux-app http://oremacs.com/2016/03/16/counsel-linux-app/

I've tried a couple, but I always end up back to Alt+F2 in XFCE, since most of these launchers always start so slowly in comparison, and I never use any other features (more than once) than starting Firefox/Emacs/Terminal …