tl;dr => keep your apps from falling asleep with KeepAwake
If your Heroku app doesn't get traffic which justifies you spending $0.05/hr (~$34/mo) and you run it on 1 dyno, you've probably noticed that it seems to fall asleep. If your app doesn't get traffic within ~10 minutes of its last request and you are running on 1 dyno, Heroku will expire your resource and allocate it elsewhere. The issue with this, especially if you're running anything with Ruby, is that it will take a significant amount of time to re-allocate that resource to your application and spin it back up.
One solution (with Ruby) is to create a rake task which performs an HTTP GET and then use Heroku's Scheduler Add-On to run the task every 10 minutes, preventing your app from falling asleep! Huzzah! EXCEPT...when you perform jobs with Heroku Scheduler, it spins up another dyno, thus bringing your total dyno count to 2 (aka $0.05/hr) and leaves you with a small but obnoxious bill at the end of the month.
I created a little nodejs app called KeepAwake which performs an HTTP GET every 5 minutes on all of the domains that are saved to it, thus telling Heroku to keep those apps awake!