There appear to be a number of devs/companies working on additional and alternative clients and apps to accompany the rM, as well as pieces of software that can be run on the rM.
A curated list of projects regarding the reMarkable paper tablet is available on the awesome page of the reHackable group on GitHub.
Here are some examples below:
Desktop clients
reMarkable Connection Utility (RCU): A (paid) cross-platform client for all-in-one offline management of backups, screenshots, notebooks, templates, wallpaper, and third-party software
Slithin: A free cross platform management client for backup, notebook, template and screens. With internal tools create and edit notebooks
Template management
Command Line Interfaces
remt python command line tool.
maxio rM2svg and exportNotebook scripts.
reHackable scripts especially repush and repull scripts to send/recieve files to/from the rMarkable.
Launcher for Applications
Draft is a launcher for programs on the tablet. It can launch the default xochitl software, but also fingerterm, or alternative readers, like Plato (see below).
Oxide a launcher for programs on the tablet that was based on Draft. It is compatible with draft, but has more built in features.
remux a launcher for the remarkable that works side by side with your apps.
Fingerterm is a terminal emulator designed for touch based interaction.
Yaft yet another framebuffer terminal.
Alternative ebook readers
Plato, an alternative reader initially written for the Kobo e-reader has been ported to the reMarkable tablet.
KOReader a document viewer for E Ink devices.