Microsoft have released a new preview version of the upcoming SQL Server 2005 (codenamed "Yukon"), with the full product expected for the beginning of 2005.
The Express Edition of the product comes as a pure database engine, just like MSDE (MicroSoft Database Engine) came as a scaled down version of previous versions of SQL Server. Actually, SQL Server 2005 Express Edition is the followup product to MSDE 2000.
This "Community Technology Preview" version shows you the latest incorporated technology only - it is less stable even than a beta version (as it has undergone less testing prior to its release to the public). So install these preview bits on dedicated testing machines only. Also, take care to exactly follow the instructions on the download page for proper installation.