Open source alternatives to Fillout

By GitHub stars it is HeyForm, with 9,074 stars and a commit on 12 Nov. Choose HeyForm when you want unlimited forms and responses on a self-hosted copy, with data in your own MongoDB. Next come OpnForm (4,009 stars) and Palform (49 stars).

Yes. The code of all 3 is public under AGPL 3.0, so you can use them with no licence fee. Running one yourself costs the server and your time to look after it, and some makers also sell a hosted plan.

Start from the job you use Fillout for. Choose Palform when form answers must be end-to-end encrypted in the browser, so even the company hosting the form cannot read them. Choose OpnForm when you need unlimited submissions on the free plan, or a copy on your own server via Docker.

2 of the 3: HeyForm and OpnForm. Each tool's page says what it needs to run.

All 3 we track had a commit in the last 30 days, among them HeyForm, OpnForm and Palform. A recent commit shows the project is kept up, not that it will stay that way.

AGPL 3.0 (3). GPL and AGPL ask you to share your changes under the same licence when you give the software to others, and AGPL also counts letting people use it over a network.

The hosting. Fillout's maker runs the servers for you; with an open source tool the updates, backups, security fixes and support are yours, unless you take the maker's hosted plan. Features differ too, so check what each one imports and exports before you move your data.