You can't install packages intended for CentOS 5 on CentOS 6. If you're not trying this deliberately then you have configured an el5 repo on an el6 system and you need to remove it and in... [by TrevorH]
How do I install the nagios NRPE plugin on CentOS 5 using yum?
I've tried yum install nagios-plugins but it's only downloading a fraction of a megabyte. Then if I do a search for one of the commands that was supposed to be installed, like check_users nothing comes up.