Below is a collection of free tools every DBA should know about.
Not in any particular order.
Qs-Config is a free, PowerShell driven, utility for generating SQL Server installation configuration files. These configuration files are useful for performing unattended, or silent, installations and provided the perfect means for standardizing your server build processes across your enterprise.
SQL Sentry Plan Explorer is a free query plan analysis tool that will allow you to find the most expensive operators by CPU, I/O, or both. This tool is a must have for any DBA, in my opinion.
ApexSQL provides their ApexSQL Complete (SQL auto-complete), ApexSQL Refactor (SQL formatting and refactoring), and ApexSQL Search (SQL text and object search), completely free of charge. Each of these satisfies common needs of a DBA.
Red-gate provides their SQL Search (search within SQL Server database schemas), SQL Scripts Manager (expert and community written scripts), DLM Dashboard (database schema change tracking), and Azure Explorer (browse and manipulate Azure blob storage).
BOU’s team has compiled a great collection of scripts, posters, and ebooks. In particular I recommend the scripts; sp_Blitz, sp_BlitzCache, sp_BlitzIndex, and sp_AskBrent.
Adam maintains and publishes sp_whoisactive, a stored procedure which is a replacement for the commonly used sp_who (or the undocumented sp_who2) built-in stored procedure. It provides deep insight into active sessions on your SQL Server.
Minionware is an LLC owned and operated by the Midnight DBAs. Much of the suite of tools is still in development (as of 6/2015) but, currently, Minion Reindex and Minion Backup are freely available. Keep an eye out for further tools, such as Minion CheckDB.
Statistics Parser is a web-based tool for parsing the IO and TIME statistics which are returned by SQL Server Management Studio. It re-formats the output into easier to read tables. Provided by Richie Rump.
HammerDB is an open source database load testing and benchmarking tool for Oracle, SQL Server, TimesTen, PostgreSQL, Greenplum, Postgres Plus Advanced Server, MySQL, Redis and Trafodion SQL on Hadoop.