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:
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).
Fingerterm is a terminal emulator designed for touch based interaction.
Plato, an alternative reader initially written for the Kobo e-reader has been ported to the reMarkable tablet.