Updown.io intrigued me but I think they only use Slack as a notification channel (SMS costs money). Uptimerobot offers Pushbullet, Slack, Boxcar as notification methods.
Datadog also looks well planned out but their free tier would only allow me to build an API to get notifications about my forum being down. I am not sure I trust my programming to handle getting the notification if my forum server is down also.
I do appreciate the feedback and having these alternatives in this thread will be beneficial for other users. They too can look at them and choose the one that may fit their need best.