This page has an overview of available monitoring software packages used to monitor Linux / BSD servers, some are more applicable to LEB's, some require more system usage and might not be good for a LEB.

Nagios

- Information
Nagios is the classic enterprise monitoring solution. It offers monitoring and alerting for servers, switches, applications, and services. It alerts users when things go wrong and alerts them again when the problem has been resolved. It is also very extendable, there are a log of plugins available and writing them yourself is also very easy
Written in: perl

URL: http://www.nagios.org/

Munin

- Information
Munin monitors your servers and creates graphs of it so that you can see how things are going status wise. It has a lot of plugins available and writing them yourself is not that hard. It can also alert you when stuff reaches a certain threshold, or even shoot it into Nagios.
Written in: perl

Hosted Munin: https://hostedmunin.com/
URL: http://munin-monitoring.org/

Collectd

- Information
5dlvupg.jpg What is collectd? collectd gathers statistics about the system it is running on and stores this information. Those statistics can then be used to find current performance bottlenecks (i.e. performance analysis) and predict future system load (i.e. capacity planning). Or if you just want pretty graphs of your private server and are fed up with some homegrown solution you're at the right place, too ;). A collectd server is able to receive data from collectd clients.
Written in:

Tutorial: https://raymii.org/s/tutorials/Collectd_server_setup_tutorial_with_web_frontend.html

Openstatus

- Information
Written by LET community member NickM (Nick Moeck), OpenStatus is an easy to use and easy to customize server monitoring system that can completely replace Nagios, Monit, and most other monitoring systems. It features Monitoring RAM, CPU, disk usage, and data transfer, Graphs of data over several time periods (1 hour, 3 hours, 6 hours, 12 hours, 1 day), Monitors Apache, MySQL, or any other process, Monitor the output of any command, Alerting via email, XMPP, IRC, or any custom method and an easy to use web interface.
Written in: Python

URL: http://openstatus.nickmoeck.com

PHP Sys Info

- Information
PHPSysinfo is a customizable PHP script that displays information about your system. It is nice for a single VPS, or two, but it does not offer alerts or monitoring multiple VPS servers. It however does wat is says and it does that good.
Written in: PHP

URL: http://phpsysinfo.sourceforge.net

RayMon

- Information
Written by LET community member Raymii, this is a distributed server monitor script using Bash for the client and PHP for the server. It is very low on RAM and resources, and monitors unlimited servers including history. It can monitor services (processes), RAM, Disk and CPU Usage, updates, logged in users, load, uptime and more. It runs on Debian, Ubuntu, RHEL, CentOS and Arch. It however has no alerting or graphs.
Written in: Client: Bash only. Server: PHP 5.2+

DEMO: http://vps11.sparklingclouds.nl/status/stat.php
URL: https://raymii.org/cms/p_Bash_PHP_Server_Status_Monitor

CyberCiti.biz HTML Status

- Information
A little script which should be ran from cron. It outputs a nice little page with server info. Has no history or alerting. Site says this about it: Shell script to get uptime, disk usage, cpu usage, RAM usage, system load, etc and output the information on a single server in html format
Written in: Bash

URL: http://bash.cyberciti.biz/monitoring/get-system-information-in-html-format/

Status2k

- Information
A paid server status monitoring program. Looks very nice. Does the basic monitoring, has history and does multiserver. Costs $30 for an unlimited license.
Written in: PHP

DEMO: http://demo.status2k.com/
URL: http://status2k.com/

Amon

- Information
Also a server monitoring package, more targeted at application developers. Requires ZeroMQ and mongoDB, so it might be a bit hard on your servers.
Written in: Python

URL: http://www.amon.cx/