SolidPing
Create and run uptime checks, read results and incidents, manage status pages, with a free tier

Project Sonar DNS Enumeration API.
Sonar API is a free Development API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
A rapid API for the Project Sonar dataset.
Details from github.com.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Create and run uptime checks, read results and incidents, manage status pages, with a free tier
Website screenshots as PNG, JPEG, WebP or PDF, and page content as markdown, with a free tier
The Sonar API is provided by Cgboal (github.com). A rapid API for the Project Sonar dataset.
Yes. The Sonar API is listed in the Development category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Sonar API requires no authentication, so you can start making requests without signing up.
Yes. The Sonar API supports CORS, so front-end JavaScript can call it directly.
Yes. The Sonar API is served over HTTPS, so it's safe to use in production.