dmenu is a fast and lightweight dynamic menu for X. It reads arbitrary text from stdin, and creates a menu with one item for each line. The user can then select an item, through the arrow keys or typing a part of the name, and the line is printed to stdout. dmenu_run is a wrapper that ships with the dmenu distribution that allows its use as an application launcher.
Installing dmenu is simple:
# pacman -S dmenu
Now, you will want to attach the dmenu_run command to a keystroke combination. This can be done either via your window manager or desktop environment configuration, or with a program like xbindkeys. See the Hotkeys article for more information.