At first I wanted to make this thread about money, but quickly realized that other kinds of support are also very important and not everyone is in position when they can easily contribute to a project that way. And freedom to use software for free without any hooks is something to be celebrated.
Hell, until recently, I myself couldn't afford to donate any money. And now though I can, living and working in Poland means some amount is worth considerably less for projects than it is for me.
I started this thread, because I just donated to KDE and that made me wonder, what projects people like enough to support – even if they could use it without doing so.
As for KDE, they not only provide very flexible, powerful and customizable desktop, but also set of applications that range from good to great. Dolphin pretty much broken all file explorers for me as non other has options I got used to. Not to mention it looks nice and is comfy to use.
Krita is great. I wish more people knew about, because as much I love GIMP, it kinda bothers me that when people ask for free/linux program that can replace Photoshop it kinda bothers me, they are pointed to GIMP and not Krita. As it's much nicer for newcomers and totally dominates when it comes to drawing. I even do image manipulation in it as interface and options make it nicer for me, even if GIMP does have more options for it. I donate little sum but do so monthly.
niu.moe is little Mastodon instance I use. I donate 1$/month to help cover hosting costs.
Task Warrior – I was mentioning it in anouther thread, so won't do it here. First time I donated to them long time ago and they actually send me back some stickers via mail. Now I decided to support them bit more as it's niche product and it would be a shame to see it vanish.
As for other forms of support I try to take my time to report bugs when I found them. I wish this process was easier, but many bugs trackers aren't exactly super intuitive and finding your way takes some time.
I made some steps towards creating my own free software, but ultimately didn't create anything that is worth using and one project that is almost finished must wait until I write my master degree (which hopefully will be worth open sourcing as well).