Free monitoring tools software for linux

Monitoring is a SuperKaramba theme for system monitoring.
My first template, icons with KDE 3.4
It monitors:
- CPU usage
- Network load
- System Info

Argus Monitoring System software is a system and network monitoring application.
It will monitor nearly anything you ask it to monitor (TCP + UDP applications, IP connectivity, SNMP OIDS, Programs, Databases, etc).
It presents a nice clean, easy to view web interface that will keep both the managers happy (Red Bad. Green Good.) and the techs happy ("Ah! thats what the problem is").
It can send alerts numerous ways (such as via pager) and can automatically escalate if someone falls asleep.
Main features:
- It is open-source available at no charge.
- It has a clean and intuitive web interface.
- The web pages can easily be understood by non-technical people.
- It can generate graphs of what is going on.
- It can monitor network connectivity (Ping test)
- It can monitor TCP/UDP ports
- It can monitor a wide variety of TCP/UDP applications (HTTP, SMTP, RADIUS, ...)
- It can monitor the output or exit code of a program (Program test)
- It can monitor the content of a web page (such as a shopping cart application)
- It can monitor the authoritativeness of a nameserver
- It can monitor SNMP OIDs (such as BGP status, UPS voltage, room temperature, ...)
- It can monitor the results of SQL queries
- It can monitor itself.
- It can be extended to monitor things that the author didnt think of
- It can notify someone (or many people) when something happens
- It can escalate, and notify someone else, if things dont get fixed.
- It can not alarm for known downtime (maintenance overrides)
- It will summarize and rate-limit multiple notifications to prevent paging-floods.
- It keeps historical statistics, for analysis or SLA verification.
- It scales well and can monitor many, many things.
- It can restrict users to viewing only certain items (user "views")
- It can restrict users access to certain features (access control)
- It can support IPv6.
- It can support SNMPv3.
- It can support l10n for your native language.
- It can support redundant multi-server configurations.
Enhancements:
- This release offers improved performance and scalability, adds support for additional tests, and adds support for failover and redundancy.

Remote Monitoring Agent (RMA in short) is an auxiliary application for HostMonitor. Enterprise license for Advanced Host Monitor already includes license for 10 installations of the agent. Holders of a Lite, Standard or Professional licenses may buy an additional license for Remote Monitoring Agent separately.
HostMonitor 4.0+ can monitor remote networks using Remote Monitoring Agents (RMA). RMA is small application that accepts requests from HostMonitor, performs test and provides information about test result back to HostMonitor.
Why you may need RMA? Here are just several reasons:
RMA increases security of the network. When you have to run the tests such as CPU Usage test or Performance Counters tests on a remote Windows system, HostMonitor must be able to log in to that system with administrators privileges. Instead you may now use an agent installed on that remote system. In this case HostMonitor will not have to log on to that system at all. HostMonitor needs just one TCP port to communicate with the RMA agent (by default it uses #1055 port, however you may set an agent to use any other port).
Remote Monitoring Agent is also a very useful tool when you have to monitor two (or many) separated networks (connected through Internet). In this case installing just one instance of RMA behind the firewall in network "A" will allow to monitor entire network "A" using the HostMonitor located in the network "B" with just one open TCP port.
RMA decreases the network traffic. E.g. frequent use of "File Integrity" or "Compare Files" tests in an array of remote systems may apply significant load on the network. The more and the bigger files you test the more traffic increase you get. RMA runs locally and sends only the test results to the HostMonitor thus decreasing the amount of network traffic.
Remote Monitoring Agent simplifies network administration. You no longer need to share local drives/folders to perform tests such as File Integrity, Folder/File Size, File Availability, Count Files, etc
RMA for Linux / BSD / Solaris allows you to perform tests that HostMonitor cannot perform. For example HostMonitor cannot monitor processes that are running on Linux systems. RMA can do that.
Main features:
- All traffic between Remote Monitoring Agents and HostMonitor is encrypted.
- It is possible to customize the list of enabled tests for each of the agents (e.g. living only Count Files and UNC tests only).
- You can restrict incoming TCP connections with the list of acceptable addresses.
- With RMA Manager you may configure, restart and even upgrade agent(s) remotely.

GetMonitor project is a Python XChat script for monitoring DCC file servers requests on sharing channels and IRC servers.

Similar to AutoGet and SDFind scripts, it helps you request files, monitor queues, and search files automatically.

NAT Monitor is a tool to monitor hosts bandwidth usage in a Linux-NAT network.

A daemon collects data and clients display them (currently a GTK app with graph and a text version). It detects new hosts, saves up to 12 hours of data, and has a nice summary statistic.

FreeBSD ports monitoring project is a simple solution for monitoring what has changed in installed ports on a FreeBSD machine. It works quite well and shows me what is happening on some machines which are manage not only by me. And leaves some trace after handy portupgrade -a command.

But when many ports are upgraded at once output from this script becomes mess and it is hard to read what was added/removed and upgraded. Well it happens when change relates to two ports, neighbours in alphabetical sequence. Diff output with my modifiers becomes unacceptable. So I rewrote it and yes, it is now in Ruby.

UPS monitor is a graphical monitoring application that lets users watch a your UPSs status in real time.
Main features:
- Saves your session if UPS switches to battery power
- Warns you when power is critically low
- Lets you monitor networked UPSes
- Remembers configuration and restarts when you log on
- Extremely easy to use