Sample Databases

The Microsoft sample databases are a great way to test the product.
The main download site for pre-SQL Server 2014 is in CodePlex and the sample databases can be found here.
AdventureWorks (OLTP) and AdventureWorksDW (data warehouse) moved to GitHub in Nov 2017 and can be found here.
The WideWorldImporters sample database for SQL Server (starting 2016) and Azure SQL Databases can be found on GitHub here.