I’ve been a lichess user for over 5 years now, and it’s one of my favorite open source projects (and the first one I ever donated to)
lichess.org is a free/libre, open-source chess server powered by volunteers and donations.
Today, Lichess users play more than five million games every day. Lichess is one of the most popular chess websites in the world while remaining 100% free. Most “free” websites subsist by selling ads or selling user data. Others do it by putting all the good stuff behind paywalls. Lichess does none of these things and never will. With no investors demanding profits, Lichess staff can focus on improving the site as their only goal.
The platform is truly free (as in freedom and free beer - Lichess features • lichess.org) and is maintained by a very small team (2 if I’m not wrong) of developers who work on it full-time.
Donations go towards the servers and to pay the developers. All their expenses are also public.
Other links -
Press kit
Github
Donations page
P.S -I don’t think this is a critical and/or underfunded project, but I felt it’s worth a mention here. Naturally any additional support to the project helps them add more features, hire devs etc.