Or if you prefer, System -> Administration -> System Monitor -> Tab: File Systems, and there you have all the file systems and not just the total. This is really important if you have /home on a separate hard drive.
And if you open any directory using nautilus (and unmark all files), you can see free space available for THAT directory down in the status bar (make sure you have the status bar active tho), even if you for some reason have a separate partition for each directory, no need for memorizing what directory is mounted on what hard drive.
Enjoy!