I am looking for a desktop program that will monitor my local filesystems and pop up an alert once the usage reaches a certain threshold. I know lots of solutions for the server environment, but that would be overkill as I just want to monitor my desktop, which runs Ubuntu 12.04.
Ubuntu already includes a disk space warning alert as part of dconf.
You can configure the settings by installing the dconf-tools:
sudo apt-get install dconf-tools
Start the dconf Editor:
CRTL + F to show the find option.
Enter "disk space" and click on "Next"
You can set the free percent warning to whatever you like.
This example is from Ubuntu version 14.04 LTS but the instructions likely apply to other versions.