An eye on your system
Monitor monthly internet Transmit and Receive bandwidth usage - Linux
Zenoss - Intelligent IT Operations Management
Xplico is a Network Forensic Analysis Tool (NFAT)
Count the number of people around you by monitoring wifi signals
Raven is the legacy Python client for Sentry
PotHead
Open Source SIEM
Scalable, distributed monitoring system for high-performance computing
Batch system monitoring and archiving