Although the current version of Hatta is quite stable, it's by no means a dead project. Development continues, both fixing bugs and making Hatta more robust, reliable and compatible with web servers and browsers, and extending Hatta with new features – although the latter is done carefully. There is also some experimenting going on, as described in Development notes.
There is also work on improving and adding Translations.
hg clone http://bitbucket.org/thesheep/hatta
See Hacking for more detailed instructions.
Hatta currently doesn't have any plugin system, so developing extensions may be a little hard, but feel free to experiment with the code, and don't hesitate to share your work if you come up with something good! Read more about extending Hatta.
There are plans on adding a real plugin system on Plugin Plans.
Report bugs on the stable version's bugtracker, at http://bitbucket.org/thesheep/hatta/issues