I have zero affiliation with the group, apart from being a casual user but http://wasitup.com/ has come in handy for me quite often. Even for free, it'll do a check 20 times an hour.
https://www.cloudkick.com/accounts/signup/developer/
CK sent me a text message as soon as my host went down earlier, which gave me time to file the ticket against Linode.
Only caveat - they don't provide a signed RPM, which can cause some administrative hardship for linux users using rpm-based distributions (most of the yum operations require you to add an extra cli option to allow working with unsigned rpm's), but it's a minor complaint given that their agent isn't mandatory and doesn't need a lot of updates.