Today Microsoft released the RTM version of SQL Server 2008 R2. The trial version is available on the following URL http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=54ad6107-c552-4d47-b1a1-3f66460f71a9.
More related downloads can be found via Download Center: http://www.microsoft.com/downloads/en/results.aspx?freetext=sql+server+2008+R2&displaylang=en&stype=s_basic
Happy downloading!