Here it is from Microsoft: Currently in CTP, The SQL Server 2005 – Everywhere Edition is a compact database (5MB memory footprint, approx. 2MB!! disk footprint)

• Full referential integrity with cascading deletes and updates
• Multiple connections for foreground and background operations
• Single data file with password protection and 128-bit file level encryption
• Transaction support to commit and rollback grouped changes
• A wide range of data-types, such as UNICODE character data types, IMAGE, MONEY, and IDENTITY
• Supports database size up to 4 gigabytes
• SET Functions (aggregates), INNER and OUTER JOIN, subselect, and GROUP BY and HAVING clauses
• Scrollable and updatable cursors to provide fast and easy data access

So. Take a look at it. There is also the slightly bigger SQL Server 2005 Express Edition, which gives you nearly all the functionality of a large scale multi-user database. But when you need to do small tasks (e.g. small websites) you need to try that one.

Source: http://www.microsoft.com/sql/ctp_sqlserver2005everywhereedition.mspx