If you've installed Ubuntu Karmic since it's Beta like me, you probably have a lot of unused Linux Kernel headers, images and modules. Actually, even if you did a fresh install of the final version, you should have a few of those which you no longer use.
We've posted this command before in a larger post (and I forgot about it), but I've remembered it today when I had realized I have ~500 MB of packages which I could remove to free up some space, so I wanted to share it again with you.
To remove all the unused Linux Kernel headers, images and modules, simply run this command:
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