Shocked that nobody recommended this?! Nagios is a AWESOME open-source solution that is very customization. We have used it in a medium-sized environment (~100 servers) and I absolutely love it. It does require you to create and customize service checks, but is very powerful.
The GUI is very user-friendly and there are applications for desktop as well as Android OS to monitor your infrastructure. Not to mention the ability to send emails, or text notifications for alerting.
Nagios is worth a look if you have the time to do some tweaking/customizing.. but even has a handful of service checks ready to run "out of the box"
**Okay so this isn't exactly a solution for monitoring your servers from several different locations, although we have setup a "client-facing" Nagios server externally to accomplish this, so it can be done if you have multiple co-locations.