Today I would like to mention LibreNMS product
https://www.librenms.org/#features
It has several features like:
- Automatic Network Discovery using CDP, FDP, LLDP, OSPF, BGP, SNMP and ARP.
- Alerting Service lie email, irc etc. etc.
- API access
- SNMP walink putting devices under monitoring
- Devices monitoring
- Graphs and reporting
- Android and iPhone app
Here they are some screenshots