There is also simply telling someone involved with open source you appreciate what they do. Whether this is by email or over IRC or in-person to some contributor of the project, having someone literally say "thank you" goes a long way. I know this might sound trivial, but realize that people contributing to open source over long periods are basically either dealing with negative people or people who are cordial but still want something from you. Having someone who doesn’t explicitly want something from you and still feels like saying "thanks" shows that your hard work is appreciated in general and not in some specific instance where you fixed some bug someone needed help with. Think of it as the difference between when you do something for a loved one and then they give you a hug as thanks versus that same person simply giving you a hug out of the blue just because they care; the former hug is nice but the latter one is what makes you feel truly appreciated.