Dmenu is a fast and lightweight dynamic menu launcher for X. It can be compared to 'Quicksilver' on OS X, or 'Launchy' on Windows. With a quick keystroke, the dmenu menu appears, enabling you to type the name of a program you wish to start.
Installing dmenu is simple:
pacman -S dmenu
Now, you will need 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.