Sorry I missed this yesterday with all the excitement over the Visual Studio SP1 release (which allowed some people to finally install the SQL Server 2008 release from last week).  Yesterday also marked the release of SQL Server 2008 Express Edition, which you can download here:

http://www.microsoft.com/express/sql/default.aspx

You can read more background about it at the sqlexpress blog; if you have a previous edition of Express on your machine(s), you should read this for important setup/upgrade information:

http://blogs.msdn.com/sqlexpress/archive/2008/08/11/sql-server-2008-express-is-now-live.aspx

Note that only the core edition of SQL Server Express has been made available; you will likely have to wait a few weeks before you see the "with Tools" and "with Advanced Services" releases that I talked about the other day.