The choice of a monitoring solution is not easy! Free or commercial, required features, agentless or agent based, security aspects, etc… There are so many requirements that it’s difficult to find the best solution.
I found a page on Wikipedia with a comparison of well-know monitoring solutions. It’s not complete, products are missing, informations details is limited but it’s a good starting point: http://en.wikipedia.org/wiki/Comparison_of_network_monitoring_systems.