OpenNMS is free software to do NMS.
Another option (but it costs money) is whatsup. Just based on all the obnoxious dazzle and needless motion on their website and their silly name, I already dislike them.
Once you have your NMS installed and running, you tell your cisco routers what server to talk to and what traps you are interested in. Take a look at this Cisco document. The game goes like this:
(config) snmp-server host x.x.x.x {community string} (config> snmp-server enable traps config (config> snmp-server enable traps snmpThis would let you know about configuration changes, reboots, snmp authentication failures, and links going up and down. There are many more (you should read the tedious cisco manual).
Tom's Computer Info / tom@mmto.org