I use Wikipedia. A lot. And I have made small donations here and there in the past, usually when they are running a "fundraiser", of sorts, but that is when they seem to be in a state of "dire straits", and need donations to stay up and running. So, I went ahead and signed up for a $5 per month contribution (which I have been meaning to do for over a year now). I can easily afford a five spot per month considering how much I use the service (which is to read up on any/every subject matter that I may be interested in (short of web dev documentation, which Wikipedia doesn't really deal with)).
This feels good, but it really feels like a "you scratch my back, I'll scratch yours" situation, because I derive a lot of value from this service.