Hi!
I don’t know what’s the best place for this but I guess I’m not the only one interested in syncing my tree with online services, even though it often goes against their ToS…
So, if you have informations hints about these, please post here or tell me what would be a better place for it 
FamilySearch
Their API is well documented at FamilySearch Developer Center — FamilySearch.org and even though they’re mentioning “App Approval Considerations”, I couldn’t find anything in their terms preventing from using it, as does getmyancestors.
MyHeritage
Even though their API is documented at https://www.familygraph.com/ and https://www.familygraphql.com/ their ToS is pretty clear that they wouldn’t like the average Joe to use it…
Except with our express prior written consent, you must not copy or store electronically all or part of our Website or its contents, or make available, distribute, sell or offer to sell all or any part of the Website or its contents, or systematically download content and data from or through the Website to make or populate another database for any purpose. Scraping, crawling and/or harvesting of any data and any pages from the Website without our explicit prior written consent is strictly prohibited. Running scripts to automate any activities on the Website, including but not limited to photo upload, photo colorization, color restoration, and/or any other photo enhancements and manipulations, is strictly prohibited.
I’ve requested an application key a few months ago but never got any answer…
In the meantime, I’ve listened a bit to their Family Tree Builder which has its own API but also gives access to the familygraph one.
Geneanet
There’s no real official api for Geneanet even though they have a GPL’d (outdated) wordpress plugin using a json API.
They also have an upload API for sending pictures along with a GEDCOM files, which is used by their own Geneanet Upload as well as Ancestris.
Their (unsupported anymore) app also uses a json API and can download the trees you have access to as a custom database whose format is fairly easy to parse.
Therefore, the user (including Premium Members) undertakes:
- to only access the Site using an ordinary web browser and solely to carry out personal or professional research;
- not to share his/her login details, which are strictly personal, so as to share his/her Geneanet access with other people;
- not to use a temporary email address;
- never to use a web robot, for any reason. A “robot” shall be deemed to be any software tool designed to reproduce the actions of a human user with a view to entering data, migrating such data, mass downloads, load or performance tests and/or “stress tests”. The use of any automatic access tool is expressly prohibited;
- not to bypass or try to bypass functionalities related to the security of the Geneanet sites, or the functionalities related to restriction of their use or to the copying of their contents or services;
- not to extract, download or copy this data other than for genealogical purposes.
- not to use this data for commercial purposes.
So obviously such use wouldn’t be very welcome…
WikiTree has a read-only API too.
And there are Browser extensions for citing data from the big players in their Apps section too. Look for WikiTree Sourcer.
FindMyPast search and hinting:
A JavaScript WikiTree Browser Extension was announced 23 Oct 2022.
Maybe the GitHub project offers hooks for driving searches that could be called by Gramps