SQL Server Reporting Services is available as part of all editions of SQL Server including Express, Standard, Developer, Workgroup, Web and Enterprise.
There is no separate license required for reporting services, unless it is running on a separate server. As per SQL Server licensing model, a separate license is required for each server on which any component of SQL Server is running. So, if you decide to host the reporting services on a separate server than your SQL Server, you may need an additional license for reporting services.
Free edition of SQL Server Reporting Services
SSRS can be obtained free as part of SQL Server Express, which is a free edition. There are certain limitations on the free Express edition of SQL Server. However, the restrictions are mainly applicable for SQL Server Express database components. However, since SSRS use ADO.NET which includes all drivers for connecting to Oracle, MySql and other database systems, you can run your reports free against any ADO.NET supported database systems, provided your Reporting services is installed as part of SQL Server Express.
Some enhanced features of reporting services are not supported on SQL Server 2008 Express edition.