Trying to think about it, I dont think I have ever donated to an open source project. Unless you count buying official CDs straight from the freebsd guys. Or time submitting bug information.
I have donated to freeware authors, I have donated to websites I find worthy, but I dont think I have ever donated to open source.
But then the only open source stuff I use are the large, old, server-ey projects which have funding from large corporate sponsors or a company behind it extracting money from support contracts.
Ah yes, there was postfix! I did donate a tad to that.
But still, I should think about the few open source small projects I still use and support them a little. After all I complain enough most open source lack polish because volunteer programmers hate the parts of the job which are usability and tying loose ends etc. So supporting the few I use which are polished (winmerge, peazip...) is a good idea. And I should get the company to donate/buy support for some they use, perhaps.