Windows Double Explorer
Windows Double Explorer is a tiny, free utility that provides two Explorer windows--each with a little more oomph than you get from Windows Explorer. Double Explorer employs a dual-pane interface, which you can view stacked or side by side. Within each pane you can create multiple tabs representing different directories. You can also drag folders to the Favorites toolbar for easy access down the road. Just uncompress the zip file and double-click the executable file--no installation required.
One important note: Windows Double Explorer works in Windows 7 only. If you have an older version of the OS, Total Commander is a good bet.