Helix Player is a 100% open source (soon to be GPL'd) GTK2-based media player that supports Ogg Vorbis, Ogg Theora, SMIL, RealText and may other datatypes. It's the foundation of the RealPlayer for Linux from RealNetworks.
Live feeds from GUADEC are available for viewing in Helix Player and the RealPlayer 10 test builds:
Main room (B1-018)
For best results viewing the GUADEC webcast, look for the latest nightly build of the Helix Player or RealPlayer (tested with RealPlayer build from 6/25)
See also: main live streaming page http://stream1.hia.no/
Presentation at GUADEC
At GUADEC, RealNetworks announced that they will be releasing the Helix Player under tri-license terms which will include the GPL in the next 30 days, in cooperation with Red Hat and Novell, who pushed hard to make this happen. In very closely related news, Novell and Red Hat are both standardizing on the Helix Player as the standard media player on their commercial distrubutions. Details on all of this are available in the press releases (Real and Novell, Real and Red Hat), as well as this piece in the New York Times. Source code and prebuilt binaries can be downloaded from the Helix Community