krunner is a mini-CLI you can open with alt-F2 by default (customizable of course). It allows searching apps or the web, opening web sites, search files, controlling music being played, or whatever really as it's extensible through plugins.
If using KDE is acceptable to you, you can learn more here: https://userbase.kde.org/Plasma/Krunner
It is based on an idea from an older launcher called Quicksilver http://qsapp.com/
These types of apps have existed in OS X for a long time. I'm kind of surprised that Linux doesn't have an equivalent yet, as pretty much any open-source dev who uses a Mac laptop knows about the above apps.
Less chance for market dominance from any one tool.
The coolest, ultra minimal yet featureful one is dmenu, which I haven't seen mentioned yet. Gnome-do, Kupfer are also good.
In doing a bit of Googling of dmenu I found this
http://alternativeto.net/software/dmenu/?platform=mac
Which may be helpful for others as a general way to find alternatives for a given piece of software on other platforms.