I've also been thinking a lot about sustainable open source software, mostly with respect to my own project, rinohtype.

I hate ads and block them whenever possible, and I believe most people hate them as well. I also have this feeling that the concept of advertising somehow doesn't rhyme with open source. It just doesn't feel right. Open source is about freedom and advertising is about being in-your-face. I don't know how to word this in a better way.

People turn to ads because it seems to be the only way to make some money from their work. There must be a better way! But how? The best I can come up with is some sort of automated royalty payment system. Commercial users of the software need to pay a license fee, which is divided among the contributors relative to their contributions. This last part is the hard part. I don't think lines of code is a good measure of the value of a contribution.