SQL Server 2008 comes in many different editions. The edition you choose will depend on your requirements. If you are looking for a free database management system, you will need to choose one of the Express editions or the Compact edition. You could also try the Evaluation edition, which allows you to trial SQL Server 2008 for 180 days.
The Editions
Here are the different editions available for SQL Server 2008.
The Editions
Here are the different editions available for SQL Server 2008.