The most recent stable release is opal-1.0. It is
available for download in the stable directory.
The snapshots of development releases can be found in
Here you can see the CVS source
repository. It allows you to browse all files of Opal and view
differences between versions. The CVS (concurrent version system)
is a program for managing versions and branches of development.
More on CVS here.
The old releases are also available for download,
altough getting the development snapshot should be more wise.