Like the original MPlayer, mpv is still primarily a command-line application although it has a more advanced user interface than MPlayer that can use not only the keyboard but also the mouse for mpv’s on screen controller (OSC).
However, this OSC is still not a full-featured GUI, and there are a number of front-ends available, which use GUI widgets of Qt, GTK, or some other widget toolkit to give mpv a more complete graphical interface.

The following are all open source front-ends of mpv (based on "libmpv" or the command-line version of mpv) which try to provide more features and more user-friendly interface than mpv, and/or better integration with various operating systems or desktop environments.

Baka MPlayer - media player on Windows, Linux, and macOS although macOS version requires user to compile from source, with Qt5 widgets, written in C++.
Its main goal is uncluttered, simple design.
Its development stalled in January 2017 in favor of another mpv frontend by the same developers, Mochi Player, which is not quite finished yet.

Deepin Movie - for Linux - Written by and default video player for the Chinese Deepin distro and desktop environment with Qt5 widgets, written in C++.

Celluloid (formerly GNOME MPV) - for Linux - based on GTK, written in C. Its goal is to be a simple GTK-based graphical interface for mpv that meets the GNOME Human Interface Guidelines.

IINA - macOS 10.10+ media player with native macOS Cocoa interface, written in Swift.
It is a full-featured native macOS graphical interface for mpv that makes use of new features in the most recent versions of macOS.
mpv config file and script system are also integrated.

Kawaii-Player - Linux and Windows 10 - media player and media server with Qt5 widgets, written in Python 3.
Its goal is to not just be a multimedia player but also an audio/video library manager and portable media server and torrent streaming server/player.

Media Player Classic Qute Theater (mpc-qt) - Linux and Windows media player with Qt5 widgets, written in C++.
Its goal is to reproduce and ultimately improve upon the functionality of Media Player Classic Home Cinema (mpc-hc), a Windows-only program, as a cross-platform mpv-based multimedia player that also works on Unix-like operating systems like Linux.

mpv for Android - Android media player with native Android interface, written in Kotlin.
Its goal is to be a fully functional, full-featured port of mpv (which primarily targets desktop operating systems) to the mobile operating system Android.

mpv.net - Windows media player with native Windows interface, based on Microsoft .NET Framework, written in C#.
Its goal is to provide the standard mpv OSC interface on Windows along with a customizable Windows context menu, C# scripting, and a Managed Extensibility Framework (MEF) for addons.

OvoPlayer - Linux and Windows music player that supports many backends, based on LCL widgetsets, written in Pascal.
Its goal is to be a flexible audio player that supports as many audio engine backends like mpv as possible.

SMPlayer - full-featured, cross-platform and skinable with advanced features and YouTube and Chromecast support that can use MPlayer or mpv.
Available for Microsoft Windows, Linux and macOS, and written in C++ with Qt4/Qt5
Sugoi Player - media player on Windows (that might work on Linux and macOS but those are untested) forked from Baka MPlayer, with Qt5 widgets, written in C++.
It aims to improve upon and continue development of an mpv frontend based on Baka MPlayer, since Baka MPlayer’s development stalled in January 2017.

xt7-player-mpv - Linux media player with Qt5 or Qt4 widgets, written in Gambas 3 (a dialect of BASIC).
Its goal is usability, and a variety of extra features like YouTube and SHOUTcast integration, media tagging, library and playlist management, as well as adding more features beyond that.
Like the original MPlayer, mpv is still primarily a command-line application although it has a more advanced user interface than MPlayer that can use not only the keyboard but also the mouse for mpv’s on screen controller (OSC). However, this OSC is still not a full-featured GUI, and there are a number of front-ends available, which use GUI widgets of Qt, GTK, or some other widget toolkit to give mpv a more complete graphical interface.
The following are all open source front-ends of mpv (based on "libmpv" or the command-line version of mpv) which try to provide more features and more user-friendly interface than mpv, and/or better integration with various operating systems or desktop environments.
Baka MPlayer - media player on Windows, Linux, and macOS although macOS version requires user to compile from source, with Qt5 widgets, written in C++. Its main goal is uncluttered, simple design. Its development stalled in January 2017 in favor of another mpv frontend by the same developers, Mochi Player, which is not quite finished yet.
Deepin Movie - for Linux - Written by and default video player for the Chinese Deepin distro and desktop environment with Qt5 widgets, written in C++.
Celluloid (formerly GNOME MPV) - for Linux - based on GTK, written in C. Its goal is to be a simple GTK-based graphical interface for mpv that meets the GNOME Human Interface Guidelines.
IINA - macOS 10.10+ media player with native macOS Cocoa interface, written in Swift. It is a full-featured native macOS graphical interface for mpv that makes use of new features in the most recent versions of macOS. mpv config file and script system are also integrated.
Kawaii-Player - Linux and Windows 10 - media player and media server with Qt5 widgets, written in Python 3. Its goal is to not just be a multimedia player but also an audio/video library manager and portable media server and torrent streaming server/player.
Media Player Classic Qute Theater (mpc-qt) - Linux and Windows media player with Qt5 widgets, written in C++. Its goal is to reproduce and ultimately improve upon the functionality of Media Player Classic Home Cinema (mpc-hc), a Windows-only program, as a cross-platform mpv-based multimedia player that also works on Unix-like operating systems like Linux.
mpv for Android - Android media player with native Android interface, written in Kotlin. Its goal is to be a fully functional, full-featured port of mpv (which primarily targets desktop operating systems) to the mobile operating system Android.
mpv.net - Windows media player with native Windows interface, based on Microsoft .NET Framework, written in C#. Its goal is to provide the standard mpv OSC interface on Windows along with a customizable Windows context menu, C# scripting, and a Managed Extensibility Framework (MEF) for addons.
OvoPlayer - Linux and Windows music player that supports many backends, based on LCL widgetsets, written in Pascal. Its goal is to be a flexible audio player that supports as many audio engine backends like mpv as possible.
SMPlayer - full-featured, cross-platform and skinable with advanced features and YouTube and Chromecast support that can use MPlayer or mpv. Available for Microsoft Windows, Linux and macOS, and written in C++ with Qt4/Qt5
Sugoi Player - media player on Windows (that might work on Linux and macOS but those are untested) forked from Baka MPlayer, with Qt5 widgets, written in C++. It aims to improve upon and continue development of an mpv frontend based on Baka MPlayer, since Baka MPlayer’s development stalled in January 2017.
xt7-player-mpv - Linux media player with Qt5 or Qt4 widgets, written in Gambas 3 (a dialect of BASIC). Its goal is usability, and a variety of extra features like YouTube and SHOUTcast integration, media tagging, library and playlist management, as well as adding more features beyond that.
Mpv (media player)