"Observium is an autodiscovering PHP/MySQL based network monitoring system focused primarily on Cisco and Linux networks but includes support for a wide range of network hardware and operating systems."
"By using (included) Eventtriggers.exe (in Server 2003), you can automatically start a batch file or VBS script which performs a custom action or captures useful information. You can also use a script to perform correction actions on the machine, preventing the problem from reoccurring. Sometimes you just need to be notified when certain events are created in the Event Viewer, so we can use a script to send an e-mail alert. For example, you can create a trigger that monitors the event logs for low disk space events and if such events occur, you can run a script that removes any temporary or unnecessary files to resolve the low disk space condition, and/or send an e-mail alert telling you about it."
Free tools like Xirrus Wi-Fi Inspector and the Xirrus Wi-Fi Monitor Gadget for Windows 7 with links to other third-party systems/network admin tools. Very cool!
"Simple Network Tester's agents run on PCs located at key locations. You command them to send live test data through your network when you need it. Run tests from your office using Simple Network Tester's user-friendly web pages. Monitor the health of your network just by clicking a few links." Tests jitter to simulate VoIP calls and has a free command-line version to download. $29 per company after 15-day trial.
Download link to Dell OpenManage System Administrator Managed Node installer that allows you to view hardware, including RAID, status on a Dell server from within Windows. It also adds logging to the event logs (which can then be monitored) for RAID-related errors and warnings.