I've been looking for a new means of getting alerts from my Nagios (server monitoring) for some time. Previously, I used SMS notifications from Nagios, but at upwards of 5p per credit, given the way a flakey link can result in 10 or 20 notifications in a short space of time, this was starting to get quite expensive. It also doesn't fit very well with the fact that SMS can cost more to receive abroad and, generally, I spend more time in places with a reliable WiFi connection that with a good mobile signal - especially when visiting family at Christmas.
What would seem appropriate here, then, is some kind of push notification service working over data rather than SMS. Notify My Android doesn't seem as well regarded these days, but Pushover looks really good. I particularly like that you're provided with an e-mail address which you can just plug in to things like Nagios to have e-mails turn up as push notifications, without having to script their API.
I'm 24 hours into their 7 day trial, and so far so good. Their app is nicely featured including mute/unmute of sound and vibrate notifications, and quiet hours.