Taskbar++ is a software that allows you to sort (move) the buttons of the Windows taskbar by Drag & Drop. It's for the people who care about a buttons' position such as "The mail program needs the leftmost!!", "Internet Explorer should be next to Explorer", and so on.
That means, if you care about the order of programs on the taskbar this applications is for you, free of course.
Update: Microsoft has changed the way the taskbar works when they released the Windows 7 operating system. The new system introduces several new features that make taskbar management a lot easier and comfortable.
It is now for instance possible to pin programs directly to the taskbar, so that they are always available there. You can start programs then with a click on the taskbar icon.
Even better, you can arrange the pinned icons any way you want, which is exactly the functionality that Taskbar++ provided for Windows XP. All you need to do is drag and drop the icons to their new location on the taskbar to arrange them the way you want. Windows saves the position automatically, which means they stay in the selected position until you change it again. You can pin programs to the taskbar by right-clicking their taskbar icon when they are open and selecting the pin to taskbar option from the context menu.
The new taskbar introduces another new feature that is called Jumplists. You can now right-click any taskbar item, and if it supports Jumplists, you see a context menu with actions. Some examples are the recently visited sites in Microsoft's Internet Explorer, the write message page in Outlook, or a private browsing link in the Firefox web browser.