Best Free Network Monitoring Applications
Network monitoring software can be quite expensive. Prices range from a few dollars per month for computer, and can go up to hundreds of dollars per month for enterprise-level applications. Fortunately, there are several free network monitoring applications as well, and they provide a set of features that is good enough for most people's needs. Here are some of the best freeware monitoring tools out there.
a) Icinga

Icinga may be a name that's difficult to spell (at least for me) but it is a tool that's got lots of great features. First of all, it is open source software, which means that it doesn't include any nasty modules that would spy your network traffic.

Lots of high profile companies utilize Icinga; Audi, Debian, Vodafone and Adobe are just a few of the big corporations that are using Icinga to monitor their networks.

You can use this software to check the availability of your network resources, no matter if we are talking about local area networks, or complex networks that spread across several worldwide locations. You can set it up to notify people of outages, and generate complex reports that focus on your network data.
b) Nagios Core

Nagios Core is probably one of Icinga's top competitors. In fact, Icinga include several modules that will help Nagios' users make the transition to Icinga much easier.

This doesn't mean that Nagios doesn't have several aces up its sleeves, though. A key feature is its ability to isolate and solve network problems, coupled with a proactive network device checker, which tries to determine potential problems before they appear.

Nagios' alert system is very flexible; the application can send you emails or SMSs whenever something goes wrong.

There is a drawback, though, and it may be a deal breaker for some of you: Nagios Core is free for a single user. If you run a small home or business network you should be fine, though.
c) Observium

Observium is an auto-discovering network monitoring tool. It supports a wide range of operating systems, servers, devices, and so on. New features and updates are delivered every six months for free application users. However, if you want to get access to new features and fixes as soon as they are released, you will have to pay about $250 per year - a small fee if you are running a big network.

Observium provides great-looking reports and an easy to use user interface. The only thing that's lacking at the moment is the ability of sending alerts if something goes wrong with your network. Sure, you can run Nagios Core alongside Observium, but let's hope that the tool will receive this much-deserved feature in the very near future.
d) Ntop

Ntop is the fantastic combination of a top-class network monitor and a beautiful, simple, easy to understand GUI. If you want to research your network traffic deeply, down to the exact type of protocol, source and destination, Ntop is definitely one of the best choices.

You can sort network traffic according to its IP addresses, display real-time network traffic, Geolocate hosts on a map, monitor SNMP devices, and so much more. Then, when you are done analyzing your data, you can create great-looking HTML 5/Ajax-based reports.
I hope that you like these free network monitoring tools. There are many more out there, of course, but I did my best to present the ones that are definitely worth the download.