RANCID (Really Awesome New Cisco confIg Differ) is a versioning system for your switch and router configs. It uses either CVS or Subversion to store each new version of your configuration files. As it gathers and stores the configs for each of your devices, it runs a diff against the previous version to see what, if any, changes have been made. When it detects a change, it sends out an email with the details of that change to an address of your choosing. With RANCID, you'll know whenever a change has been made by your NOC team.
Because RANCID runs via a crontab entry, you can control how often it logs in and checks your configurations. If you are a stable shop and rarely make changes, you might have RANCID check once a day. If you are a more dynamic NOC and make changes frequently, you can set RANCID to check hourly or as often as is appropriate for your company.
One of the neat features of RANCID is that it includes a looking-glass server. You can take a quick peek at all the routes in your organization and search for any elements that are out of sorts when you suspect a routing problem on your network.
RANCID supports gear from most of the big networking vendors, including Cisco, HP ProCurve, Juniper, Foundry, and several others. It is known to work on Linux, BSDs, Mac OS X, and Solaris.
Top free open source tools for network admins: OpenNMS and CactiOpenNMS has a place in every enterprise. It's a highly scalable network monitoring system that is completely open source software. A single server can monitor hundreds of thousands of network interfaces and produce nice graphs for metrics such as bandwidth usage, CPU, memory, and more.
You can set thresholds that indicate when a device is busy or down and receive a notification via email, SMS, IM, and so on. Of course you can have separate logins for each of your NOC team, and you can set up an on-call schedule so that notifications go only to on-duty team members. OpenNMS also has an escalation handler, so if the level-one NOC techs don't take care of an issue right away, an engineer or manager can be notified to oversee issue resolution.