I'm one of the developers who worked on Libretto. A couple of things we could use help with if somebody wants to jump on this:
* Google Compute Engine support (Difficulty: moderate)
https://github.com/apcera/libretto/issues/13
* Virtualbox headless (Difficulty: easy)
https://github.com/apcera/libretto/issues/17
* vCloud Air support (Difficulty: moderate)
https://github.com/apcera/libretto/issues/18
* More unit tests
Suggestions/comments/feedback are welcome!