Cacti is extremely well written, well presented, and infinitely customizable. There really is no comparison to this tool in either the open source or commercial world.
Nagios
Nagios is a surprisingly mature network monitoring framework that's been in active development for many years. Written in C, it's just about everything that system and network administrators could ask for in a monitoring package. The Web GUI is fast and intuitive (although it's even better with the contributed Nuvola style), and the back end is extremely robust.
As with Cacti, there is a very active community supporting Nagios, and plug-ins exist for a massive array of hardware and software. From basic ping tests to integration with plug-ins like WebInject, you can constantly monitor the status of servers, services, network links, and basically anything that speaks IP. I use Nagios to monitor server disk space, RAM and CPU utilization, FLEXlm license utilization, server exhaust temperatures, and WAN and Internet link latency. I even use it to ensure that Web servers are not only answering http queries, but that they're returning the expected pages and haven't been hijacked.
Network and server monitoring is obviously incomplete without notifications. Nagios has a full e-mail/SMS notification engine, and an escalation layout that can be used to make intelligent decisions on who and when to notify, which can save plenty of sleep if used correctly. In addition, I've integrated Nagios notifications with Jabber, so the instant an exception is thrown I get an IM from Nagios detailing the problem. The Web GUI can be used to quickly suspend notifications or acknowledge problems when they occur, and can even record notes entered by admins.
As if this wasn't enough, a mapping function displays all the monitored devices in a logical representation of their placement on the network, with color-coding to show problems as they occur.
The downside to Nagios is the configuration. The config is best done via command line and can present a significant learning curve. As with many tools, the capabilities of Nagios are immense, but the effort to take advantage of some of those capabilities is equally significant. '
But don't let the complexity discourage you -- Nagios has saved my bacon more times than I can possibly recall. The early-warning systems provided by this tool for so many different aspects of the network cannot be overstated. It's easily worth the time investment. I've written several Nagios plug-ins, including one that monitors a wide variety of APC hardware, and they've paid me back many times over.