Finder!

Double Commander is a cross platform open source file manager with two panels side by side. It is inspired by Total Commander and features some new ideas.

XYplorer is a file manager for Windows. It features tabbed browsing, a powerful file search, a versatile preview, a highly customizable interface, optional dual pane, scripting engine, and a large array of unique ways to efficiently automate frequently recurring tasks. It’s fast, light, and portable.

Nautilus (Files) is a file manager designed to fit the Gnome desktop design and behaviour, giving the user a simple way to navigate and manage its files.

QTTabBar is an extension for Windows Explorer that brings tabbed browsing to Microsoft's file browser. It comes with a number of nifty features including a favorites function for launching folders and applications, groupings of folders that can be opened together, tooltip preview functions for a number of file types, and others. It is also highly customizable and hotkey-enabled.

Dolphin is a file manager for KDE focusing on usability. Has three view modes: Icon, Detail and it used to have Column View like in Finder (it is not available in current version due to problems with maintaibility). Supports FTP, SCP and other protocols. Highly configurable.

Commander One is dual-pane file manager for OS X created in Swift, which enables you to manage your files in the best possible way.

Thunar is a new modern file manager for the Xfce Desktop Environment. Thunar has been designed from the ground up to be fast and easy-to-use. Its user interface is clean and intuitive, and does not include any confusing or useless options. Thunar is fast and responsive with a good start up time and directory load time. Thunar is accessible using Assistive Technologies and is fully standards compliant.

One Commander (previously bitCommander) is a Windows file manager (Windows 7, 8 and 10) with improved Miller Columns (OSX Finder's column view) and two-panel support. It simplifies file management with intelligent file representation, relative file dates (shown as days since creation), ToDo notes in any folder, batch file processing without interrupting for copying errors, and much more.Windows Store version: https://www.microsoft.com/store/apps/9NBLGGH4S79B

muCommander is a cross-platform file manager that will run on any operating system which supports Java. It features a Norton Commander style interface to allow easy manipulation of files with many keyboard shortcuts. Pre-compiled builds are available for Mac OS X, Windows, Linux, Solaris, *BSD, etc.

Path Finder is a standalone Finder replacement that takes the Finder's familiar interface and adds numerous powerful features and interface innovations to help anyone be more productive on Mac OS X. It features:

XtraFinder add tabs and extra features to the Finder.

"Dual pane file manager and SFTP/FTP client for Mac OS XForkLift is designed to be the fastest, most powerful file manager for the Mac. Connect to FTP or SFTP servers, manage your Amazon S3 account or your mobile devices over Bluetooth. All these features are beautifully integrated into a sophisticated, yet easy to use Cocoa based interface. Dual-pane view will dramatically increase your productivity along with state-of-the-art features like live preview, spring loaded folders and any-to-any workflow. All this to be topped by tabs, to keep your workspace nice and clean." »

Explorer++ is a free file manager for Windows. Available on Windows XP and above, it features the same familiar interface as Windows Explorer, while introducing several enhancements and improvements for a much richer file browsing experience.

TotalFinder brings tabs to your native Finder and more! Other features include dual-panel mode and showing folders before files.