Stan is now linked to NumFocus, a 501(c)(3) nonprofit organization that serves several other open-source software projects, including NumPy, Julia, Jupyter, and others:

Stan can now accept tax-deductible contributions through this foundation. If you’re interested in donating to Stan, you can contact us directly or just go through the donation page. Stan is a worthy cause: our software is free and, as users can attest, we also give a lot of free and open help through the stan-users list. We also have an active developers list with contributors from many countries. By supporting Stan, your organization is helping to make this state-of-the-art Bayesian data analysis program even better. You’re pushing forward the frontiers of science and helping your own work as well as others’.

Here’s the description of NumFocus:

The mission of NumFOCUS is to promote sustainable high-level programming languages, open code development, and reproducible scientific research. We accomplish this mission through our educational programs and events as well as through fiscal sponsorship of open source data science projects. We aim to increase collaboration and communication within the scientific computing community.

Sounds good to me!