https://altstore.io is the big one. You might want the AltStore fork SideStore (you can do the weekly reinstall without a computer, https://sidestore.io). Other tools exist, like https://sideloadly.io and https://appdb.to.

You use sideloadly to install any ipa you want. If you don't have a developer account it will sign the application using a key with the validity of seven whole days! (instead if you have a developer account it will be valid one year, and don't forget to pay the 99€/year ransom)