First of I’d like to say: “Sorry Heroku”.
If you guys want me to take this down again, or if this is against your terms of use, I’ll gladly do so.
I’ve just pushed my evil PingMachine to Github. 
PingMachine is a (very) simple Rack application, which, given SITES as a comma-separated string, will fetch each of the sites every 10 seconds or so.
Why is this evil? Well, Heroku will sleep your application if it hasn’t been active for a while (if you only have one (free) dyno). I don’t want this for my free applications, so I setup yet another Heroku app just to ping the others.
PingMachine does so, by first sleeping 10 seconds and then spawning a thread for each site. These threads will fetch the page via curb and just throw away the result. Whilst it’s curbing away, it’ll return 200 OK and a list of the sites back to you. An important note here, is that the PingMachine itself is on this list, resulting in it doing it all over again… and again… and again…
Now all my sleepy Heroku app have been awaken - and will stay awake until PingMachine dies (or is rebooted and forgotten, in which PingMachine itself will fall asleep 
)