Salesforce Setup Audit trail:

Salesforce Setup Audit trail

Login History:

How to view login history of an organization in Salesforce?

Event Monitoring:

Event Monitoring in Salesforce

Lightning Usage App:

How to view Page Performance by Browser and Other New Metrics in the Lightning Usage App?

Salesforce Optimizer:

Salesforce Optimizer

Health Check:

Health Check in Salesforce

Portal Health Report:

https://www.infallibletechie.com/2014/12/portal-health-reports.html

Apex Exception Notifications:

Apex Exception Email in Salesforce

API Usage Notification:

API Usage Notifications in Salesforce

Analytic Snapshots in Salesforce:

Analytic Snapshots in Salesforce

Transaction Security in Salesforce:

Transaction Security in Salesforce

Salesforce Identity Provider Event Log:

Salesforce Identity Provider Event Log

Salesforce Outbound Messaging Delivery Status:

Salesforce Outbound Messaging Delivery Status

Feed Tracking:

Feed Tracking in Salesforce

Field History Tracking:

Field history tracking in Salesforce

Force.com Security Source Scanner:

How to check the code quality in Salesforce?

Salesforce Debug Logs:

Debug Logs in Salesforce