Free vs Paid network monitoring tools

What James said above.  It really depends on your environment, and what (and how much) you’re wanting to monitor.  A one-man shop running a 24/7 online payment portal will have different needs than a 200-user company that has a single server used only to store Excel spreadsheets.  And a multi-national company needing up-to-the-minute reporting and monitoring for servers, routers, firewalls, and network connections spread out in different countries is completely different from both of those.

That said, make a list of what is important to monitor for you, and then compare that list to the different monitoring packages out there.

There are also some programs out there that are free up to a certain point, then become paid, PRTG one of those that immediately comes to mind.