[staslabs]

http://www.mikrotik.com/thedude.php

The Dude network monitor is a new application by MikroTik which can dramatically improve the way you manage your network enviroment. It will automatically scan all devices within specified subnets, draw and layout a map of your networks, monitor services of your devices and alert you in case some service has problems

http://www.mikrotik.com/thedude.php

Some of it's features:
The Dude is free of charge!
Auto network discovery and layout
Discovers any type or brand of device
Device, Link monitoring, and notifications
Includes SVG icons for devices, and supports custom icons and backgrounds
Easy installation and usage
Allows you to draw your own maps and add custom devices
Supports SNMP, ICMP, DNS and TCP monitoring for devices that support it
Individual Link usage monitoring and graphs
Direct access to remote control tools for device management
Supports remote Dude server and local client
Runs in Linux Wine enviroment, MacOS Darwine, and Windows
Best price/value ratio compared to other products (free of charge)

http://www.mikrotik.com/thedude.php