I get a NOKEY warning from RPM, or I get a gpg signature error when using yum!

The NOKEY warning is not really a problem. It won't prevent you from doing anything. (The yum error, though, will usually prevent you from installing software.) 

If you'd like to resolve it, do the following command to get the correct key for the site you're downloading from:

You must be root to do any of this.

• Red Hat and Fedora: 
  rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY*
  
  
• rpm.livna.org: 
  rpm --import http://rpm.livna.org/RPM-LIVNA-GPG-KEY
  
  
• FreshRPMs:
  rpm --import http://freshrpms.net/RPM-GPG-KEY-freshrpms
  
  
• DAG:
  rpm --import http://dag.wieers.com/packages/RPM-GPG-KEY.dag.txt
  
  
• ATrpms:
  rpm --import http://atrpms.net/RPM-GPG-KEY.atrpms
  
  
• NewRPMs:
  rpm --import http://newrpms.sunsite.dk/gpg-pubkey-newrpms.txt
  
  
• Dries (RPMForge):
  rpm --import http://apt.sw.be/dries/RPM-GPG-KEY.dries.txt
  
  
• JPackage:
  rpm --import http://www.jpackage.org/jpackage.asc
  
  
• kde-redhat:
  rpm --import http://kde-redhat.sourceforge.net/gpg-pubkey-ff6382fa-3e1ab2ca
  
  
• PlanetCCRMA:
  rpm --import http://ccrma.stanford.edu/planetccrma/RPM-GPG-KEY.planetccrma.txt

NOTE: If you install my yum configuration, most of these keys are installed for you automatically.