A community technology preview of the first service pack for SQL Server 2005 is now available. The CTP process, which was used for SQL Server 2005, proved to be such a good source of customer feedback, that the SQL Server team is continuing to use it with service packs.
You can download the CTP from here:
The biggest change in SP1 is support for database mirroring.
There are also a number of enhancements to SQL Server 2005 Express Edition including SQL Server Management Studio Express (SSMSE), support for full-text catalogs, and support for viewing reports via report server. These changes are included in SQL Server Express with Advanced Services. You can also download SSMSE seperately.