Taskix is a free utility that allows you to reorder the buttons in your Windows taskbar in-place using your mouse. If you have "Group similar taskbar buttons" set, it also works inside stacked groups! Simply extract the files from the ZIP into a folder of your choice and start the EXE. It is most straightforward to use, it is fast and uses very few system resources. You also have the option to configure it to be restarted with Windows.