15 GB sounds like a decent amount.
Have you deleted your APT cache? That's all the packages you've ever
downloaded. You can do that withsudo apt-get cleanor somewhere in the
Synaptic package manager too if you wish to do it graphically.
Besides that you've probably collected a fair number of kernels, you only
need one so you might want to uninstall the older ones, at 150 megs a pop.
The logs probably don't take a huge amount of space but if they do and you
have no use for them, feel free to zap them.
