Located at www.fedora.us
As the name suggests, the FedoraExtras project provides extra packages for your FedoraCore installation that don't come with it by default.
It is accessable via Yum or APT. See instructions on their site
Because of licensing issues, certain packages including but not limited to mplayer, xine and xmms-mp3 cannot be placed in FedoraExtras. You might therefore want to give http://rpm.livna.org a try.
It is considered a bad idea to mix packages from FedoraExtras and other FedoraRepositories.