Useful tools

DBAToolsCommunity driven SQL Server PowerShell module, which continously gets more functions and features.
Ola Hallengren The maintenance tool to help you automate the maintenance you absolutely need!
sp_WhoIsActive A comprehensive activity monitoring sp, which is a helpful troubleshooting partner
Brent Ozar Blitz Discover your database servers possibilities for improvement.
Diagnostic Information QueriesMagnificent DMV Queries by Glenn Berry, which will provide you with information you did not even thought you wanted.
DBAChecksYou probably need to validate your environment, and this is a good tool for it.
Microsoft Update CatalogGood to have should you need to patch servers offline.
SSMS 18SQL Server Management Studio, the tool for the database management persons.
Azure Data Studio Azure Data Studio (previously SQL Server Operations Studio), a tool focusing on getting an overview and documenting stuff.
DuckDuckGo The search engine that doesn’t track you.
List not in any particular order.

References

Execution Plan ReferenceSQL Server Execution Plan Reference
Wait Statistics LibrarySQL Server Wait Types Library
Latch Classes Library SQL Server Latch Classes Library

Versions and updates for SQL Server

SQLServerUpdates | The most recent updates for SQL Server

SQL Server 2019 (150) | DocsBuild Versions
SQL Server 2017 (140) | DocsBuild Versions
SQL Server 2016 (130) | DocsBuild Versions
SQL Server 2014 (120) | DocsBuild Versions
SQL Server 2012 (110) | DocsBuild Versions
SQL Server 2008 R2 (105) | DocsNo longer supported
SQL Server 2008 (100) |DocsNo longer supported
SQL Server 2005 (90) |DocsNo longer supported