How about letting users flattr Stack Overflow (or other Stack Exchange sites)?
I am not talking about flattring individual users, question, comments or answers. I am suggesting that the whole site could be flattred.
My reasoning: I like to block ads, even on Stack Overflow. I realize that ads are economically important to Stack Overflow. I would personally prefer to micropay for Stack Overflow instead. Letting me flattr Stack Overflow would let me block ads with a clean conscience.