In Xubuntu/Xfce the xfpanel has an add-on for that (Free Space Checker).
Is there something similar for lxpanel or otherwise in LXDE/Lubuntu - or maybe a program with a similar service for all Ubuntu family or even for all Linux?
In fact a good answer to what I was asking here for -- something in LXDE/Lubuntu that would permanently display free space as Xfce does in the panel -- is coming from the file browser which always displays free space in the lower part of the window: it's true for PCMan, Nautilus and Thunar. So simple, but I don't think there's something better in Lubuntu.
Disk Usage Analyzer
sudo apt-get install baobab
Conky is the best doing that, here is a easy way to set up a awsome conky to your desktop http://browse.deviantart.com/?q=conky+colors#/d41qrmk