Paessler is developer of network monitoring tools they have some great tools like Network 
Monitoring, Traffic Grapher and Web server stress tool.
If you have a computer network and want to track your network then you must have network monitoring tools. These tools will help you track and make it run smooth. This will find any problem or error in your network. Here are two free tools to track you network one is Network Monitor and second is Traffic Grapher.
PRTG Network Monitor: This tool will ensures the availability of network components while also 
measuring traffic and usage. It saves costs by avoiding outages, optimizing connections, saving time and controlling service level agreements (SLAs).
Avoid bandwidth and server performance bottlenecks
Deliver better quality of service to your users by being proactive
Reduce costs by buying bandwidth and hardware based on actual load
Increase profits by avoiding losses caused by undetected system failures
Find peace of mind: As long as you do not hear from PRTG via email, SMS, pager, etc. you know everything is running fine and you have more time to take care of other important business.
Download Network Monitor
PRTG Traffic Grapher: Traffic Grapher is an easy to use Windows software for monitoring and classifying
bandwidth traffic usage. It provides system administrators with live readings and long-term usage trends for their network devices. The most common usage is bandwidth management, but you can also monitor many other aspects of your network like memory and CPU utilizations.
Avoid bandwidth and server performance bottlenecks
Find out what applications or what servers cause the bandwidth traffic
Deliver better quality of service to your users by being proactive
Reduce costs by buying bandwidth and hardware according to actual load
Both these free tools are limited to manage 10 sensors. If you want to manage more then 10 sensors then you will need to buy commercial editions.
