alttab is X11 window switcher designed for minimalistic window managers or standalone X11 session.

(see man page for details)

Unlike task switchers integrated in most simple window managers (WM) or dmenu-like switchers, alttab features visual interface and convenient tactile behaviour: press modifier (Alt) - multiple switch with a key (Tab) - release modifier. Also, it's lightweight and depends only on basic X11 libs, conforming to the usage of lightweight WM.