I've once used a php shell called PHP Shell (yes, not very distinctive but that was its name, and I think there is more than one with this name), installed it on Heroku, could execute commands except root ones, but a few months later noticed they've deleted it.
I could use wget and curl, for instance, which means I could remote request (maybe ddos if I had a lot of shells) My plan was to maybe create a cloud of these shells for myself to control, and use it either for denial-of-service'ing, port/vulnerability-scanning, or massive computational power if I ever had the need.