Ask HN: What free tools or websites do you donate to?

Wikipedia was asking for donations and it got me thinking. I use a lot of free tools and web apps that don't bombard me with advertising, yet I rarely donate. There isn't a lot of spare money right now for me to make donations, but I'm beginning to make a personal list of all these useful resources I utilize on a daily basis. Eventually I would like to be able to support them all in some small way.

(Obviously, there are charities and other organizations doing a lot of good in the world that could also use contributions. I prefer to keep these two groups entirely separate.)

I'm curious what tools other people find so incredibly useful that they donate to them?