The thing is that yesterday I noticed that my root partition was a bit on the crowded side (90+%). I though it could be because of /var/cache/apt/archives/, where all the installed .deb files reside, and started purging some unneeded installed packages (very few... I only install what I need). However, I decided to double check, and Filelight has given me the clue:
Some utter disaster in a printing job filled the /var/spool/cups/tmp/ with 1.5GB of crap! After deleting it, my root partition is back to 69% full, which is normal (I partitioned my disk with 3 roots of 7.5GB (for three simultaneous OS installations, if need be), a /home of 55GB, and a secondary disk of 250GB).
Simple problem, simple solution.