Lots of threads on this, but I understand that the search function isn't working right.
Openview, Tivoli, Unicenter, Nagios, Big Brother/Sister, NMIS, JFFNMS, OpenNMS, N-Vision to name a few.
First, decide what you want to have the system do, and what you want to see in it. Then find one what works. We have N-Vision (more of a systems monitor than network), and I use NMIS, Nagios and Cacti. Each does slightly different things.