sudo apt-get clean will remove all of the downloaded packages used by apt-get/synaptic to install programs. these are perfectly safe to remove since you presumable won't have to install whatever again, but even if you did, its just another download away.
sudo apt-get autoclean will remove any partially downloaded packages used by apt-get/synaptic, also safe to remove.
you can manually see these files in /var/cache/apt/archives/, you can also delete them from that location manually as root.
also in synaptic if you click on the status bar it should split up packages into installed, installed (local or obsolute)
, installed (auto removable), not installed, and residual config.
residual config and installed (auto removable) are generally safe to remove, but don't deleted installed (local or obsolute) because I know i have a couple stuff in there which I would not remove.
For a more detailed response go here http://ubuntuforums.org/showthread.php?t=140920
And read some of the following posts concerning steps 4&5 which should only be attempted if you know what you're doing.