Speaking as a long time open source contributor, it is even better to contribute bug reports (detailed, replicable), assistance to folks on the users mailing lists, and code (if you are able).
Donating money to open source organizations is great, as there is almost always some overhead, but contributing in kind is even better!
That said, if you want to donate money, here are links to donate directly to a few open source foundations, off the top of my head:
I already do that--of course, one could always do more. Additionally, I never said a pledge had to be money--10% time would be great as well, i.e. FOSS fridays every other week or something.
With the donations, it would of course be up to the pledger, but I was thinking something more directed with higher bang for the buck. Some ideas there would be for specific tasks that never seem to get done, or via bounties, or for student contests.
In general though, I'm thinking more on a company basis rather than individual. My thought was that if a company could make a public pledge, they'd be more likely to embed it in their culture. Of course, it may be hard for existing companies to do it, so I'm thinking it may be more appropriate for startups.
In a previous job (security system tech), we could donate an install to charity - it would still show up in our numbers (for pay grades and bonuses and whatnot), but the money we would have made for that particular install would be donated. "Volunteering" had a fantastic effect in pulling the whole company together, and a lot of the time the money went toward somebody in the local community.
I would love to work for a company with a similar culture, where I could donate some number of hours per week/month to working on FOSS.
I wonder if the two could be combined. Give the money with a purpose. Surely most projects will have some long-standing tasks which have never been attempted or completed due to lack of time. The project's legal entity might not want to pay someone to work on the project directly (or there might not be such an entity), but to someone wanting to invest into the project that seems a useful option.
Donating money to open source organizations is great, as there is almost always some overhead, but contributing in kind is even better!
That said, if you want to donate money, here are links to donate directly to a few open source foundations, off the top of my head:
http://www.apache.org/foundation/contributing.html
http://www.python.org/psf/donations/
https://my.fsf.org/donate