Raphael's Flattr FOSS is a good idea: Make it really easy for users to pay tips for free software and maybe users who otherwise wouldn't express their appreciation will.
The way it splits your monthly payment amoung everyone you've Flattr'd that month is smart; it takes money out of the picture when deciding whether to click on a Flattr button.
I think it would be more honest, though, if rather than taking 10% of everything (which would be just absurd and indefensible if this were used on a large scale, and whiffs faintly of pyramid scheme even now), Flattr took one share of your monthly payment (as if you automatically Flattr'd them each month). That would provide even more incentive to Flattr lots of other things. And if lots of people were Flattring lots of things, Flattr would still make plenty of money.