Use a bit of command-line magic to pop up reminders

Here's a tidbit for the more technically inclined user of Mac OS X (in otherwords, somebody who can deal with the terminal and shell scripts): You can create "cron" jobs to pop up periodic reminders! For example, I have my development system configured to pop up a reminder every afternoon to remember to check in my files.

Use Cronnix (or if you're brave, edit your crontab file manually) to run your reminder script whenever you wish. In my case, I have a line in my crontab file to invoke my "remind_cvs" script:

30 16 * * * /Users/dwood/bin/remind_cvs
Here's the script:
#!/bin/sh
pushd ~/Development/Watson/Current
echo "---- Reminder to check in some files! ---" > /tmp/cvs_output
cvs -n -q update | sort >> /tmp/cvs_output # Show me what files are modified
open -e /tmp/cvs_output # Open this file in TextEdit
popd
There are more ways for cron to interact with you, such as Running an AppleScript. For instance, this will just pop up a dialog in the Finder at 5:30 PM every day.
30 17 * * * /usr/bin/osascript -l AppleScript -e 'tell Application "Finder" to display dialog "Did you check in your files?" '
Although for basic alarms, there's always Nicholas Riley's Pester.