You can use
Growl to create your own notifications, assuming you've installed the included
growlnotify extra (
installation instructions). Here is an example of how to notify yourself every 10 minutes about something useful like "dont forget to drink something." Open Terminal and if your shell is the bash shell, then run the following command at the prompt:
for ((i=0; i<1000; i++)); do sleep 600s && growlnotify \
-s -m "dont forget to drink"; done
This will remind you every 10 minutes to go get a drink.
[
robg adds: To kill the command, just hit Control-C. You can change the delay, the message, the loop length, etc.]