SQL Server 2005 Compact Edition - CTP/RC1
UPDATE 4 (2007-01-21): Links from this post removed, go see this post for SQLce 2005 RTM info.
UPDATE 3: I've received some complaints about confusion with regards to this post, so I've changed the title.
UPDATE 2: The runtime is currently available as an RC1 release; look for the true RTM in January 2007.
UPDATE: The links are currently broken; I have asked what the delay is.