IPMonitor is what I use, from DeepMetrix.
http://www.ipmonitor.com/
I monitor about 40 production servers with it, using various methods. It can chech HTTP and HTTPS connectivity. It can verify if Services are running, like MSSQLSERVER service. It can do a drive space check.
There are custom alert, with varying levels of alerting. E-mail alerts; NET SEND popup alerts. Event Viewer logging alerts. It can trigger other programs when an alert is detected. It can restart services if they are detected to be off.
It is a great, stable product. It has its own SMTP engine for the e-mail alerts. You can set up groups with different monitors in them, then set up different alert types and contacts per group, so you only alert the right people when alerts that they would care about occur.
I've used it since 2000, and it is perfect. HTML interface, and light. I run it on a beefed-up Compaq workstation.
hope this helps