Microsoft released SQL Server 2005 Service Pack 1 (SP1) two weeks ago, and the release includes major features that you'll want to take a look at. One of the most important parts of the release, of course, is support for Database Mirroring, which was pulled from the SQL Server 2005 release to manufacturing (RTM) last fall at the last minute.
Related: SQL Server 2005 SP1
Another major addition is SQL Server Management Studio Express, a set of GUI management tools for SQL Server Express. These tools are a major upgrade from SQL Server 2000 MSDE, which didn't include any integrated set of GUI management tools.
SP1 also introduces SQL Server 2005 Express Edition with Advanced Services. The name met with great fanfare from journalists like myself, who are often paid by the word, although some folks wonder whether Microsoft had a contest to see who could come up with the longest name. The primary difference between earlier SQL Server 2005 Express Editions and the Advanced Services model is the inclusion of support for SQL Server Reporting Services. The Reporting Services-enabled version is still free, so this edition raises all sorts of interesting options for customers who want an entirely free version of SQL Server for true production needs, albeit on a lower-scale than what SQL Server Enterprise and Standard Editions support. I'm not sure why Microsoft created a new SKU instead of simply adding Reporting Services as an install option for SQL Server Express. It will be interesting to see how this SKU might develop compared to the vanilla-version of SQL Server 2005 Express.
SP1 also extends SQL Server 2005 Reporting Services (SSRS) with a new .NET Data provider and a new MDX Query Designer that directly support enterprise reporting against SAP NetWeaver Business Intelligence.
You'll also want to make sure to download the latest version of SQL Server 2005 Books Online (BOL). The April version of BOL was updated for SP1 and can be downloaded from the SP1 Web site. The following quote from the BOL download page is important, so I wanted to draw your attention to it:
"Starting with the April 2006 update of SQL Server 2005 Books Online, a separate Books Online for SQL Server Express is no longer available. SQL Server 2005 Books Online is the primary documentation for SQL Server Express. After installing Books Online, you can use the SQL Server Express filter in the Contents pane to hide the content that does not apply to SQL Server Express edition. For more information about SQL Server Express documentation, see the topic "Using SQL Server Express Books Online."
You can download SP1 and related resources, including the latest version of BOL.