Indeed, HNN moved to haskell.org and is growing.
Firstly, the code is no longer evolving at github. There are at least 2 other persons working on HNN for the moment (Thomas Bereknyei and Jorden Mauro — thank you guys for joining the project) thus we decided it was high time having a bit more than a little github place. So we now have a darcs repository, a mailing list and a trac wiki / bug report system.
As said on the trac home page, we are rewriting HNN from scratch to make it more generic and efficient. We will of course still make the API as simple as possible, fitting one of the most important goals of the original version of HNN.
Stay tuned !