I vote Zabbix. Here's why.
1) Free but offers paid support if you need it
2) Can use agents, snmp or simple checks like ping
3) Agents can be extended with your own scripts and such. If a check isn't built in you can add it. For example, I added a very simple script for checking of MySQL replication had stopped or failed.
4) Templates, makes it easy to add a metric and create a trigger based on that metric to any host attached to that template
5) Triggers can be configured to minimize false positives (multiple dropped packets before sending an alert.
6) You can graph item, group of items or an aggregate value of items in a host group
7) Create your own maps
8) Create custom screens that group simple or complex graphs or whatever else you want onto a single page
There are some things to know about Zabbix though. You need to put some thought into items to get accurate values. Is the value you are getting from a device in bits or bytes for example. You can use custom multipliers to convert values into what you want to see.
Honestly, Zabbix is incredibly flexible and this flexibility also gives it a steep learning curve but once you get hosts entered and the templates situated the way you want it becomes very easy to add new hosts down the line. The biggest tip I can give is to make sure you spend a lot of time thinking out how to setup your templates. Zabbix includes a number of them and you'll want to customize them. One thing I found that wasn't a good idea is to make a template and then attach it to a template. It's much easier to join a host to multiple templates.
http://www.zabbix.com/
