Adventure Works 2014 (OLTP version) is a sample database for Microsoft SQL Server, which has replaced Northwind and Pub sample databases that were shipped earlier. The database is about a fictious, multinational bicycle manufacturer called Adventure Works Cycles.
The schema is for Classic Models, a retailer of scale models of classic cars. The database contains typical business data such as customers, orders, order line items, products and so on.
A bit more complex artificial database with loops.
The Northwind database contains the sales data for a fictitious company called Northwind Traders, which imports and exports specialty foods from around the world.
The pubs sample database is modeled after a book publishing company.
The venerable sakila test database: small, fake database of movies.