Yes. Every asset on this page is free to download and use. No account, no email capture, no payment required.

Yes. Most use MIT or GPLv2. Check the README in each repo for the specifics.

Watch the repo on GitHub and you will get a ping on every release. Plugins installed through WP admin update the normal way.

Fork it. If you fix something I should have caught, send a pull request. Issues are open too.