JacobPeter told you about Katapult for KDE. It's a general purpose launcher made to launch applications by typing them.
To fuel the KDE vs. GNOME debate further comes GNOME Do. Just like Katapult, applications are launched simply by typing the name. A quick key press of Super+Space (Windows+Space) brings up the dialog, at which point anything can be typed in:
You're not limited to simple actions. GNOME Do's plugin interface allows for many different actions to be added. For example, you could search Wikipedia.
You can even do track management of media players like Rhythmbox. Pressing the down arrow pops up a menu of alternate choices.
Other types of actions include Twitter, connecting to SSH, system actions, among others.
If you're a Katapult user, you can see it looks very familiar. You can get it from the author's website, and some newer distribution releases also have it available in their archives.
