RPMfusion + audio/video codecs

rpmfusion

Get the release rpm:

$ wget http://download1.rpmfusion.org/free/fedora/\
rpmfusion-free-release-stable.noarch.rpm

Check the archive's integrity via:

$ rpm --checksig rpmfusion-free-release-stable.noarch.rpm

Which should fail with:

[..] MISSING KEYS: GPG#KEY_ID [..]

Add key to your gpg keyring for checking:

$ gpg --recv-keys KEY_ID

Compare the fingerprint with the published information on the RPM Fusion key site and possibly check the web of trust:

$ gpg --fingerprint KEY_ID

If successful make the key known to rpm:

$ gpg --export -a KEY_ID > rpmfusion.asc
# rpm --import rpmfusion.asc

Check the integrity of the package for real:

$ rpm --checksig rpmfusion-free-release-stable.noarch.rpm

If it is ok install it:

# yum localinstall rpmfusion-free-release-stable.noarch.rpm

This will create config files under /etc/yum.repos.d/ and key files under /etc/pki/rpm-gpg.

Note that the # means that you have to execute those commands as root.
Also the current rpmfusion-free-release-stable.noarch.rpm is signed with the RPM Fusion Fedora 15 key but it is the right setup package for Fedora 17 as well.
For the nonfree RPM Fusion repository you have to wget the analogous setup rpm as well.
For livna.org you have to:

$ wget http://rpm.livna.org/livna-release.rpm

sudo yum install gstreamer1 gstreamer1-plugins-good gstreamer1-plugins-bad-freeworld gstreamer1-plugins-ugly

Same step, but install following for fedora 17 :

sudo yum install gstreamer gstreamer-plugins-good gstreamer-plugins-bad gstreamer-plugins-ugly gstreamer-ffmpeg libdvdread libdvdnav lsdvd

Alternatively,

sudo yum install vlc
sudo yum install smplayer