I just released v0.1 of a tiny single-header library which we use internally for monitoring and debugging hard real-time embedded applications.
We call it Legilimens 
There aren’t any open-source applications that leverage it, but we have been using it internally for over a year in a rather large deeply embedded hard real-time application with success. Legilimens allows us to sample and plot arbitrary values in real time while not interfering with the monitored application, which can be supremely useful, especially so if JTAG/SWD-based monitoring solutions are not an option.