i3 - A dynamic tiling window manager designed for X11, inspired by wmii, and written in C.
Contexts - Switch between application windows effortlessly — with Fast Search, a better Command-Tab, a Sidebar or even a quick gesture. Free trial available.
Fluxbox - Fluxbox is a window manager for X that was based on the Blackbox 0.61.1 code.
Witch - Welcome to the world of W. i. t. c. h.
Openbox - Openbox is a highly configurable, next generation window manager with extensive standards support.
VistaSwitcher - VistaSwitcher is an elegant and powerful task management utility for Windows OS.