YUM is the excellent tool for installing, deleting and querying RPM packages. The best part is YUM take cares of all dependencies of package you are trying to install.
Following are top YUM repositories for CentOS 7:
EPEL Repository:
# rpm -Uvh http://dl.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-5.noarch.rpm
RPMForge Repository:
# rpm -Uvh http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.3-1.el7.rf.x86_64.rpm
REMI Repository:
# rpm -Uvh http://rpms.famillecollet.com/enterprise/remi-release-7.rpm
Check more Repository: https://wiki.centos.org/AdditionalResources/Repositories