I'm not publishing open source software myself, but if I would start, this would be a nice license model:
Treeware is a style of software distribution similar to Postcardware, distributed by the author on the condition that users buy the author a tree.
I especially like the »conditions of use«:
You're free to use this package, but if it makes it to your production environment you are required to buy the world a tree.
You can play with things as long as you want to, but if things get serious, you have to give back.