If you want to create completely free software for other people to use, the absolute best delivery mechanism right now is static HTML and JavaScript served from a free web host with an established reputation.

Thanks to WebAssembly the set of potential software that can be served in this way is vast and, I think, under appreciated. Pyodide means we can ship client-side Python applications now!

This assumes that you would like your gift to the world to keep working for as long as possible, while granting you the freedom to lose interest and move onto other projects without needing to keep covering expenses far into the future.

Even the cheapest hosting plan requires you to monitor and update billing details every few years. Domains have to be renewed. Anything that runs server-side will inevitably need to be upgraded someday - and the longer you wait between upgrades the harder those become.

My top choice for this kind of thing in 2025 is GitHub, using GitHub Pages. It's free for public repositories and I haven't seen GitHub break a working URL that they have hosted in the 17+ years since they first launched.

A few years ago I'd have recommended Heroku on the basis that their free plan had stayed reliable for more than a decade, but Salesforce took that accumulated goodwill and incinerated it in 2022.

It almost goes without saying that you should release it under an open source license. The license alone is not enough to ensure regular human beings can make use of what you have built though: give people a link to something that works!