Types of security-relevant devices and systems that can be used to protect an organization and/or feed into a central logging server or SIEM system.
Anything that can send SNMP alerts and is security relevant
Anything that can send syslog and is security relevant
Asset management system
Change management system
Database application logs
Data loss prevention
File transfer monitor
Forensic analysis systems
Key management system
Network- and host-based intrusion detection/prevention systems
Network access controls ...