CoverflowAltTab is an Alt-Tab replacement available as an extension for 
Gnome-Shell. It lets you Alt-Tab through your windows in a cover-flow 
manner.

Originally this was a port of the CoverflowAltTab extension for 
Gnome-Shell by palatis and the recent rewrite with lots of improvements 
was done by Lusito, so a big thanks and all the glory to them!