Flatpak applications are distributed in repositories. Anyone can set up a repository for their applications, and the users only need to add the repository to the local configuration to get full integration in both the command line and the graphical installation tools (such as gnome-software).
However, most flatpak applications are available in a repository known as Flathub. See the flathub site for details about how to use it. There is also a list of all applications on flathub that you can browse.
This page lists some of the other repositories that are available, along with instructions for how to try them. If you don't have it already, you'll need to get Flatpak.
Click to download is available with Fedora 25 (and newer), Mageia 6 (and newer), OpenSUSE Tumbleweed, Debian 9 (and newer), and Arch, using GNOME Software. Some applications require version 3.22.6 or later.
These command line instructions require Flatpak 0.6.13 or newer. Once installed, applications can be launched from the usual place in your desktop. For more information on how to update and manage your applications, see the command line tutorial.
$ flatpak install --from https://linphone.org/flatpak/linphone.flatpakref
$ flatpak run com.belledonnecommunications.linphone
$ flatpak install --from https://download.mono-project.com/repo/monodevelop.flatpakref
$ flatpak run com.xamarin.MonoDevelop
$ flatpak install http://drjurf.tk/scribus-flatpak/scribus-nightly.flatpakref
$ flatpak run net.scribus.Scribus
$ flatpak install --from https://firefox-flatpak.mojefedora.cz/org.mozilla.FirefoxDevEdition.flatpakref
$ flatpak run org.mozilla.FirefoxDevEdition
$ flatpak install --from https://firefox-flatpak.mojefedora.cz/org.mozilla.FirefoxNightly.flatpakref
$ flatpak run org.mozilla.FirefoxNightly
The stable builds of GNOME applications are available on flathub, but unstable development builds are available in a separate repository. Note that these are builds directly from the latest git version and have no guarantees that they work on any given date.
First, make sure you have the unstable GNOME runtimes repository:
$ flatpak remote-add --if-not-exists gnome-nightly https://sdk.gnome.org/gnome-nightly.flatpakrepo
Now add the GNOME applications repository:
$ flatpak remote-add gnome-apps-nightly https://sdk.gnome.org/gnome-apps-nightly.flatpakrepo
You can now list the available unstable gnome applications with:
$ flatpak remote-ls gnome-apps-nightly
And install them for example like:
$ flatpak install gnome-apps-nightly org.gnome.Maps