OpenNMS Meridian http://www.opennms.org
Nagios https://www.nagios.org
The two links above look to be free, the ones below are a combination of free and paid tools.
| Solar Winds: www.solarwinds.com | Network Performance Monitor | Performance monitoring, automated network device discovery, network alerting, multi-vendor device support |
| Paessler: www.paessler.com | PRTG | Logging, bandwidth monitoring, packet sniffing, support for NetFlow Offers both a free version which is limited to 100 monitored interfaces and a paid version which is sold in increments of 500 monitored |
| Nagios: www.nagios.org | Nagios XI | Real-time event monitoring, performance and capacity planning, configuration wizards, user-specific notification preferences |
| Cacti: www.cacti.net | Cacti | Open Source product that maintains many of the same features and adds a database to store the logs as opposed to the text logs used by the popular predecessor, MRTG. |
| ManageEngine: https://www.manageengine.com/network-monitoring/ | OpManager | Paid product based on the number of sensors. OpManage touts the ability to monitor both physical and virtual environments including VMWare, Xen, Hyper-V among others. |