It depends on your monitoring needs and tech stack. New Relic has a pretty nifty monitoring product. Crashlytics is the go to for iOS apps. Testflight live has a crash monitoring and analytics product for iOS but its a bit clunky. GA you can also always view real time data and if nobody is onsite you know you have a problem (seemingly because its down). There's also free/cheaper more basic services that ping your web server to make sure your sites up and then notifies you if its down. The etsy team has a good bit of info on how they do analytics, monitoring and stats on their code is craft blog.