Most companies that do not have any network monitoring tools in place will start with OpenSource. These tools are normally what they call point tools: they focus on a certain area for network monitoring. Well known free tools are Cacti, MRTG and Nagios. Also other OpenSource network monitoring tools can be found.
If there is a need for more and deeper monitoring, companies are going to look for other systems that can do more than these point tools can offer. Now the BIG question will rise: What system is suitable for us?
The available budget and time to make a selection together with the requirements will guide you to a certain path. There are a number of tools on the market you can download easily, it also installs quickly and provides a basic overview of you network. You probably not have a lot of time to evaluate all the software, so you stick to the software that gives you the best impression. If you continue to use the software you will encounter black holes in what you want to monitor. Digging deeper into the system, it turns out that this functionality is not included in de base system. You need to upgrade or you need an addtional module.