I’ve been using these two tools a lot lately:
df – Display the amount of free space on your disks
watch – Run a command repeatedly, refreshing once every 2 seconds by default
My work involves a large amount of logging, so disk space needs to be monitored fairly closely. I use these utilities in conjunction with each other to keep track of how quickly I’m running out of disk space.
I find this script useful, too. However, because it recursively runs on every level below the working directory, it can take a while.