Winner mode is a global minor mode that records the changes in the
window configuration (i.e., how the frames are partitioned into
windows), so that you can undo them. You can toggle Winner mode
with M-x winner-mode, or by customizing the variable
winner-mode. When the mode is enabled, C-c left
winner-undo) undoes the last window configuration change. If
you change your mind while undoing, you can redo the changes you had
undone using C-c right (
Follow mode (M-x follow-mode) synchronizes several windows on the same buffer so that they always display adjacent sections of that buffer. See Follow Mode.
The Windmove package defines commands for moving directionally between neighboring windows in a frame. M-x windmove-right selects the window immediately to the right of the currently selected one, and similarly for the left, up, and down counterparts. M-x windmove-default-keybindings binds these commands to S-right etc.; doing so disables shift selection for those keys (see Shift Selection).
The command M-x compare-windows lets you compare the text shown in different windows. See Comparing Files.
Scroll All mode (M-x scroll-all-mode) is a global minor mode that causes scrolling commands and point motion commands to apply to every single window.