A (s)imple, (l)ight(w)eight, storage widget. It displays free storage space available on SD card on the left and free internal memory on the right. No configuration needed. It just works once you put it on your homescreen. No ads. 100% fat free. Only 20 KB small so it won't eat up your device storage space. It updates only once per minute to extend your battery life. You can force a refresh if you don't want to wait by pressing the widget. (It is also a button). This is a widget so you need to drop it on your homescreen it is not an app.
If you like more of those simple, lightweight widgets which only ask you for the permissions they realy need and don't fill up you phone with Megabytes of data even if they need to do only a small job then check out the other SLW widgets.
Open source GNU GPL 3.0
*added "you are doing it wrong" info, for people trying to "start" the widget like an app
*this also fixes a bug for devices with android > 4.x where the widget was not displayed in the widget list
*faster update after reboot