At least the following downloads are available:
Microsoft SQL Server 2008 R2 RTM - Express
Microsoft SQL Server2008 R2 RTM - Express with Advanced Services
Microsoft SQL Server 2008 R2 RTM - Express with Management Tools
Microsoft SQL Server 2008 R2 RTM - Management Studio Express

In case you didn't know the SQL Server 2008 R2 Express Edition - expands the size limit of a user database to 10GB.

Also looking at the downloads related to SQL Server 2008 R2 it seems that also a trial is available for download.