MSSQL is a database offered by Microsoft. This is not a free database but available for developers for testing.
Many web applications, data centers etc uses MSSQL database.
You can buy MSSQL database. For testing and developing download MSSQL 2008 R2 Express edition for free. SQL Server 2008 R2 180 days trail pack can be downloaded. Visit Microsoft MSSQL website for more details.