Potlatch is the classic OpenStreetMap editor, combining an efficient user interface with attractive presentation and a number of powerful features.
Until 2020, Potlatch was available on the Edit tab of openstreetmap.org. Now that the lifetime of Flash Player has come to an end, Potlatch (version 3) is now available as a downloadable app for Mac and Windows.
Download the latest version of Potlatch.
More detailed help is available within Potlatch and on this page.
Potlatch is open-source software, developed on Github at https://github.com/systemed/potlatch3. You can check out the source and post issues there.
Conversion of Potlatch to a desktop application has been generously supported by the OpenStreetMap Foundation. Thank you!
I'd like to make Potlatch available for Linux, but Adobe withdrew Linux support for AIR (the technology Potlatch is based on) a few years ago. If you're of a technical bent, older versions of AIR are available; there are some pointers on the downloads page.