Wikipedia currently lists the Development Status of the Sandcastle documentation generation software as "Stalled," citing the lack of recent releases. (There's actually contradictory information on this, though: their section on the history of the tool says the most recent release was 2008, the sidebar says the most recent release was 2010, and the GitHub project shows commits a mere 22 days ago).
The Sandcastle Codeplex page itself states that it's no longer under active development by Microsoft - it seems to have been turned over to an individual developer.
What's the actual status of this project? Is there a recommended alternative?