Here is a list of media players for various OSes that comes with "out-of-the-box" support for Ogg Vorbis.

DOS

Linux/BSD

XMMS
Rhythmbox
JuK: Open Source player for KDE with support for Ogg Vorbis, FLAC and MP3.
amaroK
Muine: Open Source player for GNOME with support for Ogg Vorbis, FLAC and MP3.
wxMusik: Open Source player with support for Ogg Vorbis, as well as MP3, WAV and AIFF. Makes use of an SQL based media libary for quick searching.
MPD: Music Player Daemon is a sound server that supports different clients. Supports many formats, among them Ogg Vorbis and FLAC
AlsaPlayer: ALSA audio player - "First player worldwide that did perfect reverse Ogg playback!"
Sonic-Rainbow
Cplay: Curses frontend for ogg123, mpg123 etc
Xine: Video player with long list of supported formats
Noatun: KDE audio player
CAJUN: Car audio system
GiantDisc: Jukebox system that can be controlled by a Palm
s.q.u.e.l.c.h.: Vorbis only player
MythTV: PVR project
Beep Media Player (a.k.a. BMP): XMMS fork that uses GTK+ 2.x and is compatible with both XMMS and Winamp skins
GQmpeg: GTK frontend for mpg123/ogg123 etc
BossOgg: Jukebox system
Gmerlin:
PyTone: Python-based jukebox player

VLC: Open Source player that support virtually every video and audio format.
Mplayer: Open Source video player that supports also many audio formats.
JOrbisPlayer: Java Player that uses the JOrbis Java decoder
jlGui: Winamp clone for Java
Zinf: Open Source player with support for Ogg Vorbis, as well as MP3, WAV and Audio CDs.
SnackAmp: Tcl/Tk player
HelixPlayer: Open source player from RealNetworks
RealPlayer: Commercial player from RealNetworks
Microsoft Windows Media Player: Freeware player from Microsoft. Runs on both Windows and Macintosh platforms.

The GP32, an arm9tdmi portable console with much hackability (gcc3 toolchain, expandable memory), has several Vorbis players available.

The Zaurus, a very flexible PDA which runs Linux, can play Vorbis files with a variety of software, including a plugin for the default media player, xmms, ogg123, mplayer, or theKompany.com’s tkcPlayer.

The Xbox Media Center is an open source project that transforms a modded Xbox with Linux into a media center and streaming client.

The Media Digital Media Player is a streaming client solution for the Playstation 2.

Conduits Pocket Player: Commercial player, free evaluation available.
GSPlayer: This is a free player.
PocketOgg: Open Source player that uses Tremor
PocketMusic: Commercial player.
The Core Pocket Media Player: (Formerly BetaPlayer) Open source media player that uses Tremor

WinAmp: very popular player supporting many formats
Foobar2000: freeware player with complex options and support for Ogg Vorbis, MP3, WAV and more.
Quintessential Player: Freeware player with support for Ogg Vorbis, MP3, WAV and Audio CDs. Also includes CD Ripping.
wxMusik: Open Source player with support for Ogg Vorbis, as well as MP3, WAV, AIFF and WMA. Makes use of an SQL based media libary for quick searching.
Coolplayer: very small Open Source player
dbPowerAmp: player suite
Sonique: player similar to WinAmp
XMPlay
Ashampoo Media Player+: Audio and video player
DeliPlayer:
MediaJukebox: Audio management
MuzikBrowser: Player designed for display on a TV screen
MusicMagic Mixer: Player with playlist management
MediaPortal: Open source home theater software
MPEG Audio Collection: Organizer for music collection

For hardware "mp3-players" that support Ogg Vorbis see VorbisHardware