How to use top to monitor a server's performance.
Improve uptime with Monit Server Monitoring. Monit will watch you system around the clock, and respond to out-of-the-norm events by following your instructions.
Use the vmstat tool to monitor your system's virtual memory usage.
Graphite is an enterprise-level monitoring tool renowned for its performance on systems with limited resources. It stores numeric time-series data and renders graphs of this data on demand. This guide provides an introduction to the installation and basic setup of Graphite together with Grafana.
Use Nagios to Monitor Services and Send Status Updates on Ubuntu 12.04 (Precise Pangolin).
Deploy Cacti, a Powerful Server-monitoring Solution That Uses SNMP to Track Resource Usage on Ubuntu 12.04.
Use Munin on Ubuntu 12.04 to Keep Track of Vital System Statistics and Troubleshoot Performance Problems
Use Logwatch to generate digests and analyze your server logs
Nagios is a widely used tool for monitoring systems and services.