TheoraSoftwarePlayers

Multi-platform

VLC Media Player: Open Source media player and streaming server that support virtually every video and audio format
Xiph Plugins for Real Player/Producer
Quicktime components for Quicktime 6 and Quicktime 7 – QuickTime and Macintosh OS X plug-ins
Mplayer: Open Source video player (some limitations)
CorePlayer: A multimedia platform for mobile and desktop computer systems
Cortado: Java applet playing ogg/theora/vorbis
Mozilla Firefox: Open Source web browser, since version 3.5 (was 3.1 beta)
Opera Web browser (closed source freeware), in experimental builds from 2007-Nov (very buggy), 10.5x versions (official support, but still had problems), and version 10.60 (2010-Jul-01) and newer (should be mature, also supports WebM/VP8 video)
Google Chrome: Web browser (supports both Theora, WebM/VP8 and H264)
Safari 3.1 and later: if the XiphQT components are installed

DirectShow filter: Adds support for Ogg Vorbis, Ogg Speex, Ogg Theora, Ogg FLAC, and native FLAC to any DirectShow-compliant player such as Windows Media Player and BSPlayer
Visonair.tv Player: Freeware player - Plays Ogg Vorbis and Theora streams

Helix Player - an open source media player for Linux, Solaris, and Symbian based on the Helix DNA Client media engine.
Totem - a Free Software (GPL-licensed) media player based on xine or GStreamer media engine.
Xine: a Free Software (GPL-licensed) media player, complete with its own media engine and a long list of supported formats.

Xine: a Free Software (GPL-licensed) media player, complete with its own media engine and a long list of supported formats. Supports Darwin/MacOS X (ppc) via the fink project.