Although these are clearly vendor specific, how the logic works is useful in implementing similar detection use cases in other platforms.

Elastic's detection rules

https://github.com/elastic/detection-rules/tree/main/rules

Microsoft Sentinel's detection rules

https://github.com/Azure/Azure-Sentinel/tree/master/Detections

and hunting queries

https://github.com/Azure/Azure-Sentinel/tree/master/Hunting%20Queries

This isn't Sentinel official but still potentially useful if you are running Sentinel.

https://github.com/BlueTeamLabs/sentinel-attack

A curated list of Sentinel KQL queries

https://github.com/reprise99/Sentinel-Queries

Awesome KQL

https://github.com/basedfir/awesomekql

FalconForce's Microsoft Defender for Endpoint Rules

https://github.com/FalconForceTeam/FalconFriday

Google Chronicle's detection rules

https://github.com/chronicle/detection-rules

Dan Lussier's ruleset for Chronicle

https://github.com/the2dl/chronicle_detection_public

Guide to Chronicle

https://www.crestdatasys.com/blogs/google-chronicle-security-threat-detection-and-hunting/

Splunk's Security Essentials detection rules

https://research.splunk.com/detections/

https://docs.splunksecurityessentials.com/content-detail/

https://github.com/splunk/security_content/tree/develop/detections

Splunk use cases from Abdullah Baghuth 

https://0xcybery.github.io/blog/Splunk+Use+Cases

The rules for Wazuh, an open source threat detection platform

https://github.com/wazuh/wazuh/tree/master/ruleset

The rules for Panther Labs security analytics platform. These are particularly cloud focused

https://github.com/panther-labs/panther-analysis

The rules for DNIF's SIEM platform

https://github.com/dnif/content

Sophos' IOC repository

https://github.com/sophoslabs/IoCs