This website offers a curated list of XMPP providers. Users can choose one and register on it by using their web browsers. But some XMPP apps went a step further and integrated this list in their registration process. Here you can find all known apps with XMPP Providers support.
Apps Supporting XMPP Providers
Would you like to integrate XMPP Providers in your app? There is an API for querying provider lists which are regularly updated.