Simple rake task pings your servers so your host (eg, Heroku) doesn't spin them down for inactivity.
Add this rake task to your Rails app's lib/tasks
directory, and set up a cron job to call it every 30 minutes.
If you're on Heroku, you can add the task rake app:keepalive
to the Heroku scheduler addon. I think they make you do it in 10-minute intervals, which is also okay.