In the US, if I have an invention I do not want to get a patent for, how do I publicly disclose it so someone else cannot get a patent for it

When something becomes available to the public either through implicit or explicit disclosure, then it becomes part of the prior art for patent purposes. The disclosure can be a publication for everyone to access on a webpage, technology-related journal publication, or otherwise available to the public. See 35 U.S.C. § 102(a)(1). Therefore, any of the above public disclosures or other means may make it part of the prior art.  Even though you want your invention to be open-source, you can still financially incentivize it in the form of intellectual property licensing. If this is open-source software, then you can license your software for free as long as the non-exclusive licensee (that is a user of license) complies with the terms of the license. Many open-source software providers are highly successful by licensing. Consult an intellectual property attorney for licensing your open-source work. Disclosure: The response is general in nature and cannot be construed as legal advice. If you would like to have legal advice, then please consult any competent attorney privately to discuss your case and establish a proper attorney-client relationship. I only work through my website at https://affordabletrademarkattorney.com/