Android
The best way to install Android apps is the F-Droid store. If you haven't installed it yet, no problem: It takes less than a minute and is very easy like shown in this video tutorial. You might see a warning about installing .apk files, but the F-Droid app is very safe to install and use. Once installed, clicking the below buttons should open the installation page directly in the F-Droid app.
Conversations
Conversations is the best known Android client. You can download a gratis version of Conversations on F-Droid. If you want to support the development of Conversations, you can buy it on Google's Play Store, or donate to the project. Get it on:
Note: On some Android phones the gratis version from F-Droid does not have reliable push-notifications. If your phone is affected by this, either get the paid app from the Playstore or try one of the options mentioned below.
Cheogram
Cheogram is a fork of Conversations with a focus on gateways to other services and some additional usability features. Get the latest version on their website or on:
Note: If you have issues with push-notifications, try adding https://cheogram.com/fdroid/play-services/fdroid/repo/
as an external source under the Settings -> Repositories menu of your F-droid app. This will give you access to a version that uses the Google FCM notifications service similar to the paid app from the Playstore.
Monocles Chat
Monocles chat is another fork of Conversations with a focus on userfriendly interface improvements. Get it on F-Droid or donate to the developer by buying it on the Playstore:
Movim
Movim is a web-client that works well in mobile browsers like Firefox or Chromium. It can be installed as a PWA app by visiting a Movim instance and selecting the "Add to home screen" option from the menu of your browser. Push notifications can be enabled in the Movim settings and will be delivered through the browser's web-push feature.