It seems like Flattr, the flatrate donation service is slowly taking off. The thing is still in beta, but when you subscribe for an invite, you seem to have a good chance of getting one. Currently they require a minimum monthly fee of 2€ for donation. This donation is then split across all the things whose flattr buttons you clicked and I think 10% go to the company as fee itself.
One issue I have with the current system is that it doesn't allow multiple clicking of the flattr button, which in turn makes the distribution seem quite unfair, as you sometimes flattr a single blogpost, while at other times you might flattr a complete blog or something else that is much larger then a single blogpost. This missing feature aside, it seems like a great idea and you can already find it in the wild. I spotted flattr buttons on http://taz.de/, on Chaosradio on lawblog.de and on spreeblick.com. The Flattr webpage itself also keeps a list of things you can flattr. And as you can see I added a few flattr buttons to this blog and to Pingus, Xboxdrv and some other things.
Update: I have some invite codes for Flattr.com, if you want one, drop me a mail.