Hopefully this helps someone...
I wanted a way to automatically let users know when my server goes down and comes back up. Here's what I came up with:
1. Install PRTG network monitor or Axence. Both are free. I prefer PRTG but only because I use it at work.
2. Set up a sensor which automatically logs in to the server via HTTP or HTTPS at a set interval.
3. Set up notifications that send out an email whenever this sensor goes down or comes back up.
You get the idea. It's pretty simple. If the server is down for some reason users are notified via email. When it comes back up they will receive another email. You could also send these alerts via SMS, push, through Twitter or however else you might choose to send them.