Taskbar Shuffle is a simple, small, free utility that lets you drag and drop your Windows taskbar buttons to rearrange them. Here’s a full feature list:
1. Reorder your taskbar buttons by dragging and dropping them
2. Reorder your tray icons in the same way
3. Reorder tasks in a grouped button’s popup menu in the same way
4. Middle-click to close programs on your taskbar
5. Works with UltraMon taskbars
6. Tweak taskbar button grouping

If you run more than a few applications at a time, you’ll definitely find Taskbar Shuffle useful, so give it a try!