If you are using Ubuntu, there is a good chance you get kernel upgrades once in a while. There is no reason, after you’ve booted the new kernel and verified that everything is working properly, why you should not delete all older kernels. This will free up a bit of that root partition. If you are like me and mount / on a small SSD partition, every MB counts. For me the modules for each kernel version are around 160MB.
Here is a CLI one-liner to delete all order kernels, leaving only the latest on the system:
sudo apt-get remove --purge $(dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d')