Service Pack 1 for Microsoft SQL Server 2005 is now available. You can use these packages to upgrade any of the following SQL Server 2005 editions:

• Enterprise
• Enterprise Evaluation
• Developer
• Standard
• Workgroup

Note: To upgrade SQL Server 2005 Express Edition, obtain the SP1 version of Express Edition or Express Edition with Advanced Services.

For a list of new features and improvements that are included in SQL Server 2005 SP1, review this KB Article.

Download at source.

Also:

Readme Files for SQL Server 2005 SP1 and SQL Server 2005 SP1 Express Editions and Tools

Microsoft SQL Server 2005 Express Edition with Advanced Services

Microsoft SQL Server 2005 Express Edition Toolkit

Microsoft SQL Server Management Studio Express

Microsoft SQL Server 2005 Upgrade Advisor