Below you find my most valuable SQL Server and database related links. To make it a bit easier to find what you are looking for, I have categorized the links.

You can give feedback, report bugs etc. at SQL Server feedback forum.

Performance monitoring tools

Stuff you pay for:

SQL Sentry
Quest Spotlight
SQL Monitor

Free staff that are easy and simple enough so you can tweak them and "make them your own" with the changes you make:
Erin Stellato's stuff

Free stuff that are more "closed", in the sense that complexity, hiding source code or similar gives you limited "tweakability". There is a grey line between below and above, obviously:
SQL Monitor
Database Health Monitor from Steve Stedman
Clint Huffman's PAL

Other related stuff:
Best SQL Server Performance Monitoring Tools and Software Comparison and Reviews
6 Best SQL Server Monitoring Tools for 2021
Free SQL Server Load Testing Tools

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