Cron job that shutsdown your hosted system and sends you an email to help prompt you to do it, and it's free to use forever.

0 0 15 12 * /bin/bash -c 'echo "System will shutdown in solidarity with Domain X needing to be renewed." | mail -s "Renew your domain asshole" your_email@example.com && /sbin/shutdown -h +15'

Update 15 for day, 12 for month (this goes off on Dec 15th)

I hope this wasn't too cheeky, an app seems overkill here.

Edit: if you prefer a paid service,

0 0 15 12 * /bin/bash -c 'echo "System will shutdown in solidarity with Domain X needing to be renewed. You have 2 more uses before you lose your free status and are billed 10.95 monthly for our monitoring service." | mail -s "Renew your domain asshole" your_email@example.com && /sbin/shutdown -h +15'