SQL Server 2008 Service Pack 3 - US
SQL Server 2008 SP3 contains the cumulative updates of SQL Server 2008 SP2 cumulative update package 1 to 4, and fixes to issues that have been reported through our customer feedback platforms. These include supportability enhancements and issues that have been reported through Windows Error Reporting.
Size: 304.0 - 371.0 MB

SQL Server 2008 Service Pack 2 - US  
These packages may be used to upgrade any edition of SQL Server 2008. Service Pack 2 contains updates for SQL Server Utility, Data-Tier Application (DAC), as well as integration capability for Microsoft Reporting Services with the Microsoft SharePoint 2010 Technologies. Service Pack 2 introduces support for a maximum of 15,000 partitions in a database, and includes SQL Server 2008 SP1 Cumulative Update 1 to 8.  While keeping product changes contained, we have made significant investments to ease deployment and management of Service Packs. Microsoft SQL Server 2008 Service Pack 1 is not a prerequisite for installing SQL Server 2008 Service Pack 2.
Size: 284.8 - 349.4.0 MB

SQL Server 2008 Service Pack 1 - US  
Download SQL Server 2008 SP1.
Descritption of SP1 for SQL Server 2008
Size: 15 KB - 899.5 MB

Microsoft® SQL Server® 2008 Express with Tools 
Microsoft SQL Server 2008 Express with Tools is a free, easy-to use version of SQL Server Express that includes more features and makes it easier than ever to start developing powerful data-driven applications for the web or local desktop.
This Installation requires Microsoft .NET Framework 3.5 SP1, Windows Installer 4.5 and Windows PowerShell 1.0.
Size: 218.6 MB - 449.0 MB

Microsoft® SQL Server® 2008 Express 
SQL Server 2008 Express is a free edition of SQL Server that is ideal for learning and building desktop and small server applications, and for redistribution by ISVs.
Size: 61.1 MB - 232.7 MB