Microsoft has released a final service pack for SQL Server 2008 R2: Service Pack 3. The build # is 10.50.6000.34 and you can download it here. This is essentially a rollup of all of the fixes from the 13 cumulative updates for this branch (if you want to get a complete list of fixes, start at KB #2730301). For the fixes specific to SP3, see KB #2979597. There are only two issues officially listed in addition to the cumulative update fixes; these include the security fixes from MS14-044 (also see the Release Services blog post).
There is no independent service pack release for SQL Server 2008 R2 Express; however, you should be able to apply the standard Service Pack 3 against any eligible instance. Also, Service Pack 3 will not be published using Microsoft Update.
Important note: this will be the last service release of any kind for SQL Server 2008 R2 (there will be no cumulative updates for SP3). So if you are not moving to 2012 and were waiting for some sign to deploy fixes, this may be it.
My usual disclaimer: this update is NOT for SQL Server 2008 (or SQL Server 2012). Only apply to systems where SELECT @@VERSION returns 10.50.xxxx, where xxxx is < 6000.