I've written a very annoying, unmissable Reminder.
It's a little Java-Program that opens a window which takes up almost the whole screen, containing a JTextArea with HUGE letters that change their color every 400 milliseconds, telling me that I have a meeting in 10 minutes or to take out the trash.

I use alarm-clock-applet for Ubuntu to start a shell script at the desired time, which starts the Java-Program and sends an OS-notfication saying "DO IT NOW!" after I close the window.

Code on GitHub