Just to kick off this topic: If you are running some kind of monitoring system, there will be a moment in time that the performance of certain applications will be an issue. The first thing people start to say: It's the network! The network is slow!
In my honest opinion, there is a lot of confusion in the market. This is exploited by vendors to sell you something: it is not suited for you or doesnt have the right fit for you. This needs a little bit of explanation. If you want to manage your infrastructure, Network and System Management is the mother of all: You manage you infrastructure and this can be done only by monitoring. There is a lot of confusion between the different terms being used in this market.
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.