The GNU Midnight Commander is absolutely positively the piece of software I use the most on Linux. It is a clone of the classic Norton Commander for DOS, but with certain enhancements. For one thing, it is a seamless FTP client that lets me copy files to and from my own web space, or from various third-party DOS and Linux repositories, as easily as I can copy files between drives on my own system. And I can go into an .iso image file as if it were simply a read-only directory. It's a console-based program, not a GUI, but so much the better for that. Sometimes rolling old school is best.
The Norton Commander concept with two panes (and a command line on the bottom) is by far the most efficient way to handle basic management tasks such as copying, moving, editing, viewing, and deleting files, once you know all the keyboard shortcuts and have them committed to muscle memory in your fingers. Most distros don't ship with Midnight Commander already installed, so it's always the very first thing I download when I try a new OS. If my computer is turned on, it will always have a console open with at least one tab running my beloved Midnight Commander.