MPD is designed around a client/server architecture, where the clients and server (MPD is the server) interact over a network. Thus, running MPD is only half of the equation. To use MPD, you need to install a MPD client (aka MPD interface).
This is an (incomplete) list of free and open source MPD clients.
mpc - a solid, lightweight, simple mpd client, written in C.
ncmpc - A curses client written in C++.
pms - A console client written in Go with vi-like interface.
vimpc - A curses client with vi-like key bindings.
pimpd2 - Feature-packed cli client with interactive shell and regex queries.
netjukebox the flexible media share - netjukebox is a web-based media jukebox for MPD, VideoLAN and Winamp/httpQ.
MPD.FM - A mobile-ready web client focussed on playing internet radio stations
PiFi Radio - A minimalist web client to listen to radio.
RompЯ - A mobile-ready and desktop web client
ympd - a web GUI written in C, using websockets and bootstrap.js
Cantata - A Qt client.
CoverGrid - A client for the Music Player Daemon (MPD), focusing on albums instead of single tracks
Quimup - QUIMUP is a client for the music player daemon (MPD) written in C++ and QT3 with focus on mouse handling and drag and drop.
Skympc - Yet anthoer QT(5) Client
Xfce4-mpc-plugin - Plugin for Xfce4 Panel
Xfmpc - A graphical GTK+ MPD client focusing on low footprint
M.A.L.P. - A mpd client for Android
MPDroid - A modern MPD Client for Android
ncmpcpp - A curses client written in C++ tailored after ncmpc.
ario - Another GTK based client.
gmpc(Gnome Music Player Client) - A fully featured client.
QMPDClient - Qt4 based mpd client, originally developed by Håvard Tautra Knutsen. Set of patches developed by community was then called “QMPDClient-ne”. Now it became mainline
Sonata - Client, now ported to Gtk3. At the writing time - more recent, compared to some other gtk clients.
mpdscribble - Last.fm scrobbler
There are many more clients. Please help and add them to this list.