I'd like to hear your suggestions for reputable, fairly large server monitoring services. The kind where they'll send you an alert if/when a service or server goes down, for a fee in the range of $20-100 per month. If you know an approximate number of customers your suggestion has, that would be great, since the major thing for me with these kinds of services is that most are too anonymous and hard to trust, since these kinds of businesses are relatively cheap and easy to set up in your parents' basement. I'm not interested in installed software, since I'm not at the scale needed to set up my own independent monitoring infrastructure.
I appreciate any ideas or suggestions. Thanks!