Running out of space
will give you a list of the size of each top level directory. Continue the process down one level to find the biggest folders on the system.
From your post, it sounds like you are surprised that the / partition is full, which leads me to think that it may have been created too small. If that's the case, Linode has a nifty little utility to resize ext2/ext3 filesystems. Don't forget to shutdown first if you decide you need to resize though. Resizing mounted filesystems is A Bad Thing (TM)
By default, apt keeps the .debs you download, in case you want to install them again - and sometimes you do: consider trying several different packages that do "foo". They might have similar library dependencies, and you wouldn't want to re-download all those libraries every time you "apt-get purge foo1; apt-get install foo2". Anyway, 'apt-get clean' deletes all those extra .debs. You can configure aptitude (and possibly apt-get) to do this automatically.