Dave Winer writes with regards to licensing Frontier.

1. No breakage. I want old scripts continue to run in new environments. A lot has been invested in code that runs in the Frontier environment, one of the reasons to release the kernel as source is so that those apps will run better, in more operating systems. I want to limit incentives for people to fork based on compatibility. I don't want to create a dozen semi-clones of Frontier, rather I want to incentivize people to add to the culture, add new features, fix user interface bugs, but not to break apps.

2. I want it to be possible to create a commercial business from the code base. However, I want the general rule to be that if you make an improvement to the code, you must share it on equal terms.

I think these two goals clearly imply a base license that's GPL-like, with an option for a more liberal license, for either a cash fee, or an agreement to remain compatible, or a combination of fee and agreement. This is a derivative of the MySQL license system.

This are pretty well exactly the same terms under which I want to release Jäger. I was looking at the MySQL license and it doesn't look like that great a base to start with so I may just try to throw something together very simple myself based on one of the Python licenses and then run it past a friendly lawyer.