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.