@email@example.com It's really difficult to find good alternatives which are open source.Most times they're either stupid text captchas or closed source.Here are some alternatives,not all of them perfect but maybe acceptable:
- https://captcheck.netsyms.com I just found out about it.It's open source and works with icons,so it's fast and easy to solve.
- https://wiki.hsr.ch/StefanKeller/wiki.cgi?ReMAPTCHA It's based on OpenStreetMap and improves the map data by solving captchas,so it's a win-win situation.It's also open source but unfortunately the git isn't public so you have to drop the author a email (please let me know about the response if you do)
- https://github.com/Lokno/click-captcha Is what I used on my old,proprietary sites and it was really cool.Just click on the circle and you're finished.It's open source and selfhosted.
- http://www.pluscaptcha.com Is what I currently use.Unfortunately it's closed source but it seems to be a small hobby project so privacy shouldn't be that bad there.
- https://www.keycaptcha.com Closed source but free and looks like that's also not a big company.I somehow trust the smaller ones much more.Very simple to solve.
- https://www.geetest.com/en/ Closed source and made by a Chinese company.Maybe not the best option but at least it's simple to solve and user-friendly so it's also not the worst.
- https://www.phpcaptcha.org A basic open source and selfhosted text captcha.Looks ugly but is still way less frustrating than the Google shit.
I hope I could help you with this list.I don't know if all of those are good enough for your recommendation page but maybe at least a few of them.
Generiere Instanz-Beschreibung... ... ... ... ... ERROR 418!