Each player (except for the last two) have been reviewed before here at TuxArena and there is a link to the review after the application's name.
Amarok - full review
I don't think Amarok needs an introduction. Every long-time KDE user tested it at least once, and most of KDE users use it as their primary music player, not to mention it comes by default in distributions like Kubuntu, the KDE-based branch of Ubuntu. Amarok is a powerful, collection-oriented player, with an interface which, although loved by some and contested by others, looks very good and offers as much information as it could. It comes with dynamic playlist support, cover manager, Internet services, Last.fm integration, scripting support, Internet radio support, media devices support and much, much more.
Amarok - one of the most popular and powerful music players on Linux
Rhythmbox - full review
Rhythmbox is the default audio player in Gnome and Ubuntu implicitly. Among the top features it comes with are the music library, sortable playlists, plugins, local file cover support, Last.fm integration, support for services like Jamendo or Magnatune. Another feature of Rhythmbox is the integration with Ubuntu One Music Store, the service provided by Canonical, the company behind Ubuntu (I don't know if this is Ubuntu-specific only though).
Rhythmbox is the default audio player in Gnome
Banshee - full review
Some would recommend Banshee as the first option for replacing Rhythmbox. Banshee integrates in Gnome and it comes with a full set of features, offering a media library, 10-band equalizer, Last.fm integration, cover fetching, playlist support, podcasts and a whole bunch of plugins . Banshee also offers a video library and support for playing movies and video formats. Its only problem seems to be stability though, I had some problems with random crashes and fetching local covers doesn't seem to work (Ubuntu 10.10 and Banshee 1.8.0).
Banshee is a full-featured player which also includes video capabilities
Audacious - full review
This is the GTK2 replacement for the now deprecated XMMS. It comes with Winamp 2.x skin support, an interface which resembles the Winamp 2 look and feel, support for covers, an equalizer, and more. Version 2.4 offers two view modes available in the View menu: Gtk interface and the Winamp Classic interface (screenshots below).
Audacious is a replacement for XMMS which offers two view modes: Gtk and Winamp Classic
Clementine - full review
For those who complain about the direction Amarok 2 took, Clementine can be a very good alternative to Amarok 1.4. Clementine's goal is a port of Amarok 1.4 to KDE4, and until now it's done a good job. Some of the many features of Amarok 1.4 are not yet implemented, but still Clementine offers sortable playlists, cover manager, equalizer, collection manager, file manager, Last.fm song submission. It still lacks support for scripts though.
Clementine aims to be the KDE4 port of Amarok 1.4 and does a pretty good job
Qmmp - full review
I was talking about Audacious being the GTK replacement of XMMS. Well, Qmmp is the Qt replacement for it, making it perfect for KDE users. It resembles the XMMS interface, supports Winamp 2.x skins, it allows plugins.
Qmmp - the KDE replacement of XMMS
Listen - full review
Listen is another player for Gnome, and it comes with a clean, well-organised interface as well as support for plugins, Last.fm integration, four display modes (small, normal, full and party mode), lyrics fetching, Wikipedia integration for artist and song info.
Listen - typical player for Gnome, featuring a well-organised interface
Exaile - full review
Yet another GTK application, Exaile features music collection, lyrics fetching, file browser, radio support, sortable playlist, device manager, 10-band equalizer, queue, covers support. Exaile is also highly configurable via the Edit -> Preferences menu.
Exaile is a feature-rich GTK player offering a good alternative to Rhythmbox or Banshee
Jajuk - full review
Well, when I reviewed Jajuk I was really impressed by this audio player. Jajuk is written in Java and it is a full-featured, top notch player with a neat and intuitive interface. Among the many features of Jajuk there are cover fetching, lyrics, file browser, lots of configuration options, statistics, cover manager, highly configurable interface. In my opinion, a must-try.
Decibel Audio Player - full review
Decibel can be described in one word: minimalist. Written in GTK and aiming to stay simple and intuitive, Decibel can look too simple at first, but don't be confused by that. It still offers enough features one would expect from an audio player and most of Decibel's features are available via plugins (bundled by default in it), so go to Edit->Preferences if you want to take advantage of all the features. It has three view modes (full, playlist, mini), cover fetching support, Last.fm song submission, equalizer, library, Twitter status update plugin. The interface offers a playlist and a file manager, which shows folders in a tree-view mode. Adding music to the playlist may seem at little confusing at first, you can do that by selecting a folder in the combo box to the left of the main window.
Decibel can be extended using the plugins included by default
Quod Libet - full review
Another player for Gnome with standard features and a clean, good-looking interface and support for Ogg Vorbis, FLAC, MP3, MP4, WAV and WMA formats. Quod Libet offers filters, a whole bunch of plugins (actually the plugins extend Quod Libet's functionality a lot, and there are a lot of them), several view modes.
Guayadeque - full review
I featured Guayadeque in my Linux applications with peculiar names article, and found out in the mean time that Guayadeque is a place in Canary Islands. Guayadeque brings nice features also taking a different approach regarding its interface. Features: radio, Last.fm integration, music library, lyrics, dynamic playlists, podcasts support, file browser, ratings. I think Guayadeque does a great job taking a different approach then most of the players out there.
Taking a different approach, Guayadeque is a must try if tired of usual players
Foobnix - full review
Another player for Linux, Foobnix is written in Python and uses the GTK toolkit. It offers the usual equalizer, Last.fm song submission, an online search tool and enough configuration options. The current version in Ubuntu is a little buggy, but other than that Foobnix is a good audio player.
Written in PyGTK, Foobnix offers enough features
Goggles Music Manager - full review
Goggles Music Manager is written using the FOX toolkit, a library for designing lightweight applications. It features AlbumArt support, tag editing, smart sorting, drag and drop support, Last.fm and Libre.fm song submission, SQLite database for the music collection, system tray integration.
GMM is a lightweight music player written using FOX toolkit
It looks like most of the players in this review are written in GTK, and those for KDE are only Amarok, Clementine, and JuK. This player offers features like cover manager, history, crossfading or the tag editor. JuK is very good for those who want a lightweight audio player for KDE.
JuK - a simple KDE player which has been around for years
DeaDBeeF is a player coming with a compact interface and features like an equalizer, tray integration, plugins, global hotkeys. Among the notable plugins are the OSD or the AlbumArt plugin.
DeaDBeeF offers a compact interface and an equalizer
Except for those, there are the client-server oriented ones, like XMMS2 and MPD (with clients such as Sonata or GMPC - for MPD and the ones listed here - for XMMS2), or text user interface players which can run in a terminal, like CMus (review here), mp3blaster or moc (review here).