Frequent Linux (kernel) updates clutter the free disc space left on your hard drive. If everything works with the new version, delete the old ones with this small script.

remove_unused_kernels.sh

#!/bin/sh
dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d' | xargs sudo apt-get -y purge

After this run 1,071 MB of disk space were freed on the system hdd.
Code first seen on the ubuntugenius blog.

$ dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d' | xargs sudo apt-get -y purge
...
The following packages will be REMOVED:
  linux-headers-3.11.0-24* linux-headers-3.11.0-24-generic*
  linux-headers-3.13.0-32* linux-headers-3.13.0-32-generic*
  linux-headers-3.13.0-34* linux-headers-3.13.0-34-generic*
  linux-headers-3.13.0-35* linux-headers-3.13.0-35-generic*
  linux-image-3.11.0-24-generic* linux-image-3.13.0-32-generic*
  linux-image-3.13.0-34-generic* linux-image-3.13.0-35-generic*
  linux-image-extra-3.11.0-24-generic* linux-image-extra-3.13.0-32-generic*
  linux-image-extra-3.13.0-34-generic* linux-image-extra-3.13.0-35-generic*
0 upgraded, 0 newly installed, 16 to remove and 9 not upgraded.
After this operation, 1,071 MB disk space will be freed.
...
Removing linux-headers-3.11.0-24-generic (3.11.0-24.41) ...
Removing linux-headers-3.11.0-24 (3.11.0-24.41) ...
...