Linux only: While lots of people like advanced file browsers like previously mentioned Total Commander or Path Finder, sometimes the explorer can get overwhelmingly complicated. Nautilus Elementary is a mod that compacts and streamlines Nautilus in GNOME to make file browsing easier.
Installing Nautilus elementary is fairly easy, even by Linux standards. Just open up a Terminal window and run the following commands (hitting enter after each line, and entering your password when prompted):
sudo add-apt-repository ppa:am-monkeyd/nautilus-elementary-ppa
sudo apt-get update
sudo apt-get upgrade
After it's installed hit Alt+F2 to open up the run dialog. Type in pkill nautilus to kill the currently running iteration of Nautilus, hit enter, and then hit Alt+F2 again and run nautilus to start it up again. You'll notice immediately that the window has a few buttons missing from the toolbar and that the toolbar has been condensed to fit everything on one line. In addition, the back and forward buttons are now just arrows instead of space-hogging icons with text. Nautilus Elementary is pictured above, while the old Nautilus in Lucid Lynx is pictured below for comparison purposes.
If you open up Nautilus' preferences, you'll see that there is a new tab called "Tweaks" that gives you a few more options for customising your new, simplified browser. You can enable RGBA in Compiz, toggle between a vertical and horizontal toolbar, and a few other small things. Within Nautilus, you can further tweak the window just by hitting F8, F9, and F3; which hide the toolbar, hide the left-hand navigation pane, and open up an extra pane, respectively. The extra pane is especially useful for dragging and dropping files between two folders without having to open up another Nautilus window, and the hotkeys to enable these extra panes are very convenient, letting you open them only when you need them. If you use Nautilus heavily but find it more cluttered than functional, this is a great mod to try out.
Nautilus Elementary is a free download for the GNOME desktop environment on Linux machines.