The latest public build of the next version of the Microsoft Database Engine (MSDE), SQL Server's little brother, is now publicly available. Grab the 39MB "SQL Server 2005 Express" preview download here. Please do make sure to follow the installation instructions on the download page (scroll down)!
You might also be interested in getting to know Microsoft's "SQL Server 2005 Express Manager", which is a product aimed at simplifying database management by providing features to simplify, automate, and reduce the complexity of database support and administration. Naturally, it is very closely related to SQL Server 2005 Express. The product has its own preview available as a download here.
As always, these preview software evaluation bits come without official support of any kind and no warranty is being made. Remember to run evaluation software on dedicated testing machines only.