I have been looking for a way to have a grid-like alt-tab popup menu in contrast to the default, single row popup menu.
The only extension I have found that alters the popup menu is coverflow but not in the way I want.
I use Workspace Grid for managing workspaces and it has the grid-like layout that I am looking for from the application switcher.
Does this feature currently exist?
future reference
switchPopup.js and altTab.js should be the relevant for implementing this feature, and extensions Workspace Grid and Workspace Matrix, which I believe modify workspaceSwitcherPopup.js and workspace.js, should provide startup code.
As a workaround, the activities overview gives a somewhat gridlike list. Alt+F1 invokes it, but you can change it according to this AskUbuntu answer.