FedoraCore is missing a number of important features that most desktop Linux users are looking for. There are some things that they have decided not to package for political reasons, and some are left out because there exists an alternative which is better accepted.
In order to get all this extra software, you can tell your automated package management system, be it APT or Yum, to use some extra repositories of software that is expressely packaged for your distribution.
EricRaymond has created a 
Fedora multimedia HOWTO which covers how to do all of this with the built in up2date tool.
Fedora Extras, previously known as fedora.us, is the official place for extra Fedora packages. It's as hard to get a package included in Fedora Extras as it is on the official FC CDs, and the administrators have pissed a lot of people off with statements that effectively say "It's impossible for any other repositories to exist but ours".
Complementing fedora.us is livna.org, the official unofficial companion site that contains all the stuff that Fedora aren't allowed to do, such as the MP3 plugin for XMMS, DVD/CSS code, etc.
There are a number of people that package RPMs for FedoraCore and other similar distributions (RedHatEnterpriseLinux and older RedHatLinux versions).
There are five of these repositories that collaborate closely, and they are listed below.
List of packages available from these repositories (and who is responsible for them)Mailing listBugzillaMatthias Saou)See our FreshRpms page.
Dag Wieers)The second most prolific packager behind Matthias Saou. Very general purpose. One of the only people packaging for RedHatEnterpriseLinux.
Repository homepage: http://dag.wieers.com/home-made/apt/
Axel Thimm)Originally some physics-related packages, ATrpms has become a multimedia focused repository, packaging useful things like MythTv.
However, some people don't like some of Axel's packaging policies.
Repository homepage: http://atrpms.net/
Rudolf Kastl)Repository homepage: http://newrpms.sunsite.dk/index.php?section=2
Dries Verachtert)Some esoteric packages that, in the most part, depend on packages from NewRpms, Dag and FreshRpms.
Repository homepage: http://dries.studentenweb.org/ayo/
It's clearly possible to cooperate between repositories (see below), but fedora.us don't believe in playing nice, so if you pick fedora/livna, don't use Freshrpms et al, or be prepared for things to break if you do. Most stuff that fedora.us package is also done by someone else; also remember you can install packages manually with little trouble.
A good way to start is to download FedoraFaq's yum.conf and select the repositories you want to use by uncommenting them and commenting out the others.
Then, you can install software like the xmms MP3 plugin with 'yum install xmms', and no more thinking is required.
Matt Brown adds I think you can safely live without fedora.us. I use apt for my package management, although I still use yum through the RHN applet to keep up to date with newly released packages from RedHat.
4 pages link to FedoraRepositories:
