By request, the "timeleft" script from earlier has been upgraded.
It still shows how much time is left in your day (at work or awake) as a progress bar.
However, now it reminds you that the day is over with notify-osd popups, which look really nice in Ubuntu 9.04.
It also, optionally, has an audio reminders, which are numbered ogg files. I have included a personal set of ogg files, and think you may enjoy them. If they don't fit for you, just replace the set with a different ogg files, or disable the sound notifications.
To install, simply extract the above archive in your /home/$USER/bin directory. You may need to get some dependencies, so run this just to make sure you have all of them.
sudo apt-get install python-gtk2 gtk2-engines python-pygame
It should work, just like that. However, you will want to change the times and maybe a few other settings, so have a look at the top of the /home/$USER/bin/timeleft.py file.
Thats it. Just type timeleft.py to run the program. Write it in the terminal, set it up in your gnome-session, or make a desktop icon. Whatever works for you.