The domain extension most commonly associated with non-profit organizations is ".org." The ".org" top-level domain (TLD) was originally intended for use by non-profit organizations, but it is now open for registration by any individual or entity. However, many non-profit organizations still prefer to use ".org" as it has become widely recognized as a symbol of the non-profit sector.
Using a ".org" domain extension for a non-profit organization can help convey the organization's mission, values, and purpose to visitors. It adds credibility and indicates that the organization operates in the non-profit sector. Additionally, many donors and supporters are accustomed to looking for non-profit organizations using the ".org" extension.
While there are other domain extensions available for non-profit organizations, such as ".ngo" or ".nonprofit," these extensions are not as widely recognized or commonly used as ".org." Therefore, if you're starting a non-profit organization, choosing a ".org" domain extension would be a suitable and recommended choice.