Flattr is the kind of idea that sounds great, that people love talking about - if only they could make a small donation to every website they liked - but that doesn't work because people are fundamentally too selfish. This is why ads are the only viable business model for a kind of "ephemeral website" - something you visit only briefly, derive some short knowledge or pleasure from until you follow a link somewhere el…
Most websites, and indeed open source projects are pretty faceless, and require limited interaction with it's creator. I believe, if you somehow "solve" that problem first, people will beg to donate/subscribe.