In Why I'm signing up for Gittip, a core CPython committer, Brett Cannon, mentions a great reason to reward FLOSS contributors financially:
... a way for the community to thank someone's family for letting them share their loved one with open source. For instance, tips for me would be a way of thanking my wife for letting me spend the hours I do contributing to Python in my various ways by letting me treat my wife to a night out so neither of us has to cook.
He inspired me to actually go find a bit more about this Gittip thing, after which I decided to participate as well.
In terms of money, I previously fooled around with flattr too, but stopped along the way for some reason (after a few months), so will see how long this one relationship lasts.