Cacti with SNMP is a very good single app solution to keeping an eye on server health trends.

http://cacti.net/screenshots.php

Smokeping is quite useful for monitoring network and basic http request response times.

http://oss.oetiker.ch/smokeping/

Nagios is good for actively monitoring services and alerting you when things go wrong.

I generally use those and create tons of additional monitoring tools that generate reports/charts.