Remove all but the last kernels with:
sudo apt-get purge linux-image-{3.0.0-12,2.6.3{1-21,2-25,8-{1[012],8}}}
This is shorthand for:
sudo apt-get purge linux-image-3.0.0-12 linux-image-2.6.31-21 linux-image-2.6.32-25 linux-image-2.6.38-10 linux-image-2.6.38-11 linux-image-2.6.38-12 linux-image-2.6.38-8