I was looking to make a quick reminder for myself today, and noticed I didn't have a decent alarm app on my system. I quickly went and installed an alarm widget for gnome, but alas, it will only play sounds, not show visible popups. But all was not lost! It does give the option of specifying a custom command to launch. So the answer here became zenity.
With zenity, you can launch gtk windows through the command-line. for a detailed description of it's capabilities, just man zenity.
Here's a quick example of what I did:
zenity --info --title="Train Alarm" --width=320 --text="You have 8 minutes to leave for the train. Make sure to get yourself ready."
This command opens an info window with the text I specified. I have my alarm widget run this command each day after 6, and viola! instant visible alarms!