Markus is an online grading and assignment submission platform made in house by UofT professors and used in several universities. I've used it many times myself.
For the 8 months from January 2022 to August 2022, and the 4 months from September 2023 to December 2023, I contributed to Markus's codebase in the form of volunteering, alongside other students, under the supervision of Professor David Liu. I see it as an inexpensive way of giving back to the community :)