If your /boot partition is 100% full on a CentOS/Redhat/Fedora system you can use a yum-utils package to clean up older kernels and configure yum not to keep too many versions.
First, make sure yum-utils is installed:
yum install yum-utils
Next, check the kernels that you have installed:
rpm -q kernel
kernel-2.6.32-279.el6.x86_64 kernel-2.6.32-279.2.1.el6.x86_64 kernel-2.6.32-279.5.2.el6.x86_64 kernel-2.6.32-279.9.1.el6.x86_64
And finally to keep only two of the latest kernels, run:
package-cleanup --oldkernels --count=2
To make the change permanent when installing / updating kernels via yum, edit the /etc/yum.conf file and make the following change:
installonly_limit=2